basic: split parsing of the OOM score adjust value into its own function in parse-util.c

And port config_parse_exec_oom_score_adjust() over to use it.

While we are at it, let's also fix config_parse_exec_oom_score_adjust()
to accept an empty string for turning off OOM score adjustments set
earlier.

src/basic/parse-util.h

@@ -118,3 +118,5 @@ int parse_percent(const char *p);
 int parse_nice(const char *p, int *ret);
 
 int parse_ip_port(const char *s, uint16_t *ret);
+
+int parse_oom_score_adjust(const char *s, int *ret);