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.

They were also exported but did not appear in the symbol file
or used by any of the procps binaries. They are no longer exported.

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

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

proc/pwcache.h

@@ -9,8 +9,8 @@ __BEGIN_DECLS
 // used in pwcache and in readproc to set size of username or groupname
 #define P_G_SZ 33
 
-char *user_from_uid(uid_t uid);
-char *group_from_gid(gid_t gid);
+char *pwcache_get_user(uid_t uid);
+char *pwcache_get_group(gid_t gid);
 
 __END_DECLS