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.

ps/common.h

@@ -434,7 +434,6 @@ extern int             prefer_bsd_defaults;
 extern int             running_only;
 extern int             screen_cols;
 extern int             screen_rows;
-extern time_t          seconds_since_boot;
 extern selection_node *selection_list;
 extern unsigned        simple_select;
 extern sort_node      *sort_list;