shared: add generic factory reset state apis

Let's provide a generic implementation of the systemd.factory_reset
kernel cmdline checking repart implements. Moreover add support for
leaving the factory reset state again.

This only establishes the basic APIs, it does not hook them up with
anything.

src/shared/meson.build

@@ -70,6 +70,7 @@ shared_sources = files(
         'exec-util.c',
         'exit-status.c',
         'extension-util.c',
+        'factory-reset.c',
         'fdset.c',
         'fido2-util.c',
         'find-esp.c',