util-lib: unify parsing of nice level values

This adds parse_nice() that parses a nice level and ensures it is in the right
range, via a new nice_is_valid() helper. It then ports over a number of users
to this.

No functional changes.

src/basic/parse-util.h

@@ -108,3 +108,5 @@ 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_nice(const char *p, int *ret);