util-lib: introduce parse_percent() for parsing percent specifications

And port a couple of users over to it.

src/basic/parse-util.h

@@ -105,3 +105,5 @@ static inline int safe_atozu(const char *s, size_t *ret_u) {
 int safe_atod(const char *s, double *ret_d);
 
 int parse_fractional_part_u(const char **s, size_t digits, unsigned *res);
+
+int parse_percent(const char *p);