library: procps_uptime() return value is a status

procps_uptime, previously just plain uptime() used to put the
uptime (as a double) in the first argument and return uptime
(as an int).

It meant if you ran
 myuptime2 = procps_uptime(&myuptime1, NULL);
You might get different results for myuptime1 and myuptime2 because
they are different types.

Most library calls use the return value to return the status,
procps_uptime was in the middle.

Until now.

This function will return 0 on success. If you want (for whatever
reason) uptime as an int then cast it.

All of the procps binaries didn't use the return value for uptime
except ps which set a variable to it but never used it anywhere.

proc/uptime.c

@@ -63,7 +63,7 @@ static int count_users(void)
  * Unlike other procps functions this closes the file each time
  * Either uptime_secs or idle_secs can be null
  *
- * Returns: uptime_secs on success and <0 on failure
+ * Returns: 0 on success and <0 on failure
  */
 PROCPS_EXPORT int procps_uptime(
         double *restrict uptime_secs,
@@ -91,7 +91,7 @@ PROCPS_EXPORT int procps_uptime(
         *uptime_secs = up;
     if (idle_secs)
         *idle_secs = idle;
-    return up;
+    return 0;
 }
 
 /*