systemctl: new option --drop-in for specifying drop-in filename

Previously 'systemctl edit' would only operate on
'override.conf', but users may need more than that.
Thus the new option '--drop-in' is added to allow
users to specify the drop-in file name.

Closes #25767

src/systemctl/systemctl.h

@@ -97,6 +97,7 @@ extern TimestampStyle arg_timestamp_style;
 extern bool arg_read_only;
 extern bool arg_mkdir;
 extern bool arg_marked;
+extern const char *arg_drop_in;
 
 static inline const char* arg_job_mode(void) {
         return _arg_job_mode ?: "replace";