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.

doc/procps_uptime.3

@@ -16,7 +16,7 @@
 .\" Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
 .\" %%%LICENSE_END
 .\"
-.TH PROCPS_UPTIME 3 2016-04-16 "libproc-2"
+.TH PROCPS_UPTIME 3 2016-05-01 "libproc-2"
 .\" Please adjust this date whenever revising the manpage.
 .\"
 .SH NAME
@@ -35,7 +35,7 @@ fills the location of the two double points with the current sytem uptime
 and idle time.
 .SH RETURN VALUE
 .BR procps_uptime ()
-returns a positive integer which is the uptime on success. On failure, it
+returns 0 on success. On failure, it
 returns a negative integer to one of the values defined below.
 .TP
 .B -ERANGE