crypsetup: introduce x-initrd.attach option

This option is an indication for PID1 that the entry in crypttab is handled by
initrd only and therefore it shouldn't interfer during the usual start-up and
shutdown process.

It should be primarily used with the encrypted device containing the root FS as
we want to keep it (and thus its encrypted device) until the very end of the
shutdown process, i.e. when initrd takes over.

This option is the counterpart of "x-initrd.mount" used in fstab.

Note that the slice containing the cryptsetup services also needs to drop the
usual shutdown dependencies as it's required by the cryptsetup services.

Fixes: #14224

src/cryptsetup/cryptsetup.c

@@ -228,7 +228,7 @@ static int parse_one_option(const char *option) {
                 if (r < 0)
                         return log_error_errno(r, "Failed to parse %s: %m", option);
 
-        } else
+        } else if (!streq(option, "x-initrd.attach"))
                 log_warning("Encountered unknown /etc/crypttab option '%s', ignoring.", option);
 
         return 0;