machine-id-commit: merge machine-id-commit functionality into machine-id-setup

And remove machine-id-commit as separate binary.

There's really no point in keeping this separate, as the sources are
pretty much identical, and have pretty identical interfaces. Let's unify
this in one binary.

Given that machine-id-commit was a private binary of systemd (shipped in
/usr/lib/) removing the tool is not an API break.

While we are at it, improve the documentation of the command substantially.

man/systemd-machine-id-commit.service.xml

@@ -42,55 +42,50 @@
 
   <refnamediv>
     <refname>systemd-machine-id-commit.service</refname>
-    <refpurpose>Commit transient machine-id to disk</refpurpose>
+    <refpurpose>Commit a transient machine-id to disk</refpurpose>
   </refnamediv>
 
   <refsynopsisdiv>
     <para><filename>systemd-machine-id-commit.service</filename></para>
-    <para><filename>/usr/lib/systemd/systemd-machine-id-commit</filename></para>
   </refsynopsisdiv>
 
   <refsect1>
     <title>Description</title>
 
-    <para><filename>systemd-machine-id-commit.service</filename> is a
-    service responsible for committing any transient
-    <filename>/etc/machine-id</filename> file to a writable file
+    <para><filename>systemd-machine-id-commit.service</filename> is an
+    early-boot service responsible for committing transient
+    <filename>/etc/machine-id</filename> files to a writable disk file
     system. See
     <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>
-    for more information about this file.</para>
+    for more information about machine IDs.</para>
 
-    <para>This service is started shortly after
-    <filename>local-fs.target</filename> if
-    <filename>/etc/machine-id</filename> is an independent mount point
-    (probably a tmpfs one) and /etc is writable.
-    <command>systemd-machine-id-commit</command> will then write
-    current machine ID to disk and unmount the transient
+    <para>This service is started after
+    <filename>local-fs.target</filename> in case
+    <filename>/etc/machine-id</filename> is a mount point of its own
+    (usually from a memory file system such as
+    <literal>tmpfs</literal>) and /etc is writable. The service will
+    invoke <command>systemd-machine-id-setup --commit</command>, which
+    writes the current transient machine ID to disk and unmount the
     <filename>/etc/machine-id</filename> file in a race-free manner to
-    ensure that file is always valid for other processes.</para>
+    ensure that file is always valid and accessible for other
+    processes. See
+    <citerefentry><refentrytitle>systemd-machine-id-setup</refentrytitle><manvolnum>1</manvolnum></citerefentry>
+    for details.</para>
 
-    <para>Note that the traditional way to initialize the machine ID
-    in <filename>/etc/machine-id</filename> is to use
-    <command>systemd-machine-id-setup</command> by system installer
-    tools. You can also use
-    <citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry>
-    to initialize the machine ID on mounted (but not booted) system
-    images. The main use case for that service is
-    <filename>/etc/machine-id</filename> being an empty file at boot
-    and initrd chaining to systemd giving it a read only file system
-    that will be turned read-write later during the boot
-    process.</para>
-
-    <para>There is no consequence if that service fails other than a
-    newer machine-id will be generated during next system boot.
-    </para>
+    <para>The main use case of this service are systems where
+    <filename>/etc/machine-id</filename> is read-only and initially
+    not initialized. In this case the system manager will generate a
+    transient machine ID file on a memory file system, and mount it
+    over <filename>/etc/machine-id</filename>, during the early boot
+    phase. This service is then invoked in a later boot phase, as soon
+    as <filename>/etc</filename> has been remounted writable and the
+    ID may thus be committed to disk to make it permanent.</para>
   </refsect1>
 
   <refsect1>
     <title>See Also</title>
     <para>
       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
-      <citerefentry><refentrytitle>systemd-machine-id-commit</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
       <citerefentry><refentrytitle>systemd-machine-id-setup</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
       <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
       <citerefentry><refentrytitle>systemd-firstboot</refentrytitle><manvolnum>1</manvolnum></citerefentry>