util: move percent/permille/permyriad parser into percent-util.[ch]

A good chunk of parse-util.[ch] has been about parsing parts per
hundred/thousand/ten-thousand. Let's split that out into its own file.

No code changes, just some shuffling around.

src/basic/parse-util.h

@@ -127,15 +127,6 @@ 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_unbounded(const char *p);
-int parse_percent(const char *p);
-
-int parse_permille_unbounded(const char *p);
-int parse_permille(const char *p);
-
-int parse_permyriad_unbounded(const char *p);
-int parse_permyriad(const char *p);
-
 int parse_nice(const char *p, int *ret);
 
 int parse_ip_port(const char *s, uint16_t *ret);