terminal: add systemd-subterm example

The systemd-subterm example is a stacked terminal that shows how to
use sd-term. Instead of rendering images and displaying it via X11/etc.,
it uses its parent terminal to display the page (terminal-emulator inside
a terminal-emulator) (like GNU-screen and friends do).

This is only for testing and not installed system-wide!

.gitignore

@@ -107,6 +107,7 @@
 /systemd-shutdownd
 /systemd-sleep
 /systemd-socket-proxyd
+/systemd-subterm
 /systemd-sysctl
 /systemd-system-update-generator
 /systemd-sysusers

Makefile.am

@@ -2849,6 +2849,17 @@ libsystemd_terminal_la_LIBADD = \
 	libsystemd-internal.la \
 	libsystemd-shared.la
 
+noinst_PROGRAMS += \
+	systemd-subterm
+
+systemd_subterm_SOURCES = \
+	src/libsystemd-terminal/subterm.c
+
+systemd_subterm_LDADD = \
+	libsystemd-terminal.la \
+	libsystemd-internal.la \
+	libsystemd-shared.la
+
 test_term_page_SOURCES = \
 	src/libsystemd-terminal/test-term-page.c