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/pwcache.h

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