core: embed essential units as built-in fallbacks

Compile the contents of a few essential target unit files
(graphical.target, multi-user.target, rescue.target, etc.) into the
manager binary, with comments stripped, and fall back to those built-in
copies when no fragment is found on disk. This lets the manager reach a
usable state even on a system that ships none of these unit files, e.g.
in minimal environments or one-off chroots.

On-disk files always take precedence (masks included), as the fallback is
only consulted when nothing is found in the lookup paths. The unit file
list is defined in units/meson.build and embedded via a small generator
script; 'units' is now processed before 'src/core' so the list is
available there.

The set of units and services is selected that should allow for a normal
operation of starting/stopping/restarting, of the machine. The services
all use SuccessAction=…, so they don't require any binaries to be
installed.

sigpwr.target is included, even though it doesn't do anything useful.
The same is true in normal installations. We might want to change it
do something useful there too.

units/meson.build

@@ -1070,6 +1070,33 @@ units = [
 udev_units = []
 hwdb_units = []
 
+# Unit files whose contents are embedded into the manager binary, so that it can
+# fall back to a built-in copy when the file is not present on disk. Referenced
+# from src/core/meson.build.
+embedded_unit_names = [
+        'basic.target',
+        'exit.target',
+        'final.target',
+        'graphical.target',
+        'halt.target',
+        'kexec.target',
+        'multi-user.target',
+        'poweroff.target',
+        'reboot.target',
+        'shutdown.target',
+        'sigpwr.target',
+        'soft-reboot.target',
+        'sysinit.target',
+        'systemd-exit.service',
+        'systemd-halt.service',
+        'systemd-kexec.service',
+        'systemd-poweroff.service',
+        'systemd-reboot.service',
+        'systemd-soft-reboot.service',
+        'umount.target',
+]
+embedded_units = []
+
 foreach unit : units
         source = unit.get('file')
 
@@ -1092,7 +1119,7 @@ foreach unit : units
         endforeach
 
         if needs_jinja
-                t = custom_target(
+                processed = custom_target(
                         input : source,
                         output : name,
                         command : [jinja2_cmdline, '@INPUT@', '@OUTPUT@'],
@@ -1100,9 +1127,9 @@ foreach unit : units
                         install_dir : systemunitdir,
                         install_tag : unit.get('install_tag', ''))
                 if unit.get('install_tag', '') == 'udev'
-                        udev_units += t
+                        udev_units += processed
                 elif unit.get('install_tag', '') == 'hwdb'
-                        hwdb_units += t
+                        hwdb_units += processed
                 endif
         elif install
                 install_data(source,
@@ -1110,6 +1137,11 @@ foreach unit : units
                              install_tag : unit.get('install_tag', ''))
         endif
 
+        if name in embedded_unit_names
+                # Embed either the processed output or the original file.
+                embedded_units += needs_jinja ? processed : source
+        endif
+
         if install
                 foreach target : unit.get('symlinks', [])
                         if target.endswith('/')