library: rename clashing pwcache functions

On MacOS the system already has user_from_uid and group_from_gid.
These are renamed pwcache_get_user and pwcache_get_group.

For the old library, pwcache_get_user needs to be exported
for skill.

References:
 https://gitlab.com/procps-ng/procps/issues/34

Signed-off-by: Craig Small <csmall@enc.com.au>

proc/libprocps.sym

@@ -58,7 +58,7 @@ global:
 	tty_to_dev;
 	unix_print_signals;
 	uptime;
-	user_from_uid;
+	pwcache_get_user;
 	procps_linux_version;
 local:
 	*;