parse-util: rename trivial argument for parse_pid()

The function parse_pid() trivially returns pid, hence it is not
necessary to name the argument for storing result as 'ret_pid'.

src/basic/parse-util.c

@@ -63,7 +63,7 @@ int parse_tristate_full(const char *v, const char *third, int *ret) {
         return 0;
 }
 
-int parse_pid(const char *s, pid_t* ret_pid) {
+int parse_pid(const char *s, pid_t *ret) {
         unsigned long ul = 0;
         pid_t pid;
         int r;
@@ -82,8 +82,8 @@ int parse_pid(const char *s, pid_t* ret_pid) {
         if (!pid_is_valid(pid))
                 return -ERANGE;
 
-        if (ret_pid)
-                *ret_pid = pid;
+        if (ret)
+                *ret = pid;
         return 0;
 }