tree-wide: add a helper to parse boolean optarg

This nicely covers the case when optarg is optional. The same parser can be
used when the option string passed to getopt_long() requires a parameter and
when it doesn't.

The error messages are made consistent.
Also fixes a log error c&p in --crash-reboot message.

src/shared/parse-argument.h

@@ -3,6 +3,7 @@
 
 #include "json.h"
 
+int parse_boolean_argument(const char *optname, const char *s, bool *ret);
 int parse_json_argument(const char *s, JsonFormatFlags *ret);
 int parse_path_argument(const char *path, bool suppress_root, char **arg);
 int parse_signal_argument(const char *s, int *ret);