log: include TID= field in structred log output

It always was the intention to expose this as trusted field _TID=, i.e.
automatically determine it from journald via some SCM_xyz field or so,
but this is never happened, and it's unlikely this will be added anytime
soon to the kernel either, hence let's just generate this sender side,
even if it means it's untrusted.

man/systemd.journal-fields.xml

@@ -149,6 +149,13 @@
           <literal>file:/</literal>, <literal>man:</literal> or <literal>info:</literal> URL.</para>
         </listitem>
       </varlistentry>
+
+      <varlistentry>
+        <term><varname>TID=</varname></term>
+        <listitem>
+          <para>The numeric thread ID (TID) the log message originates from.</para>
+        </listitem>
+      </varlistentry>
     </variablelist>
   </refsect1>
 

src/basic/log.c

@@ -547,6 +547,7 @@ static int log_do_header(
         r = snprintf(header, size,
                      "PRIORITY=%i\n"
                      "SYSLOG_FACILITY=%i\n"
+                     "TID=" PID_FMT "\n"
                      "%s%.256s%s"        /* CODE_FILE */
                      "%s%.*i%s"          /* CODE_LINE */
                      "%s%.256s%s"        /* CODE_FUNC */
@@ -556,6 +557,7 @@ static int log_do_header(
                      "SYSLOG_IDENTIFIER=%.256s\n",
                      LOG_PRI(level),
                      LOG_FAC(level),
+                     gettid(),
                      isempty(file) ? "" : "CODE_FILE=",
                      isempty(file) ? "" : file,
                      isempty(file) ? "" : "\n",