library: extend thread safety to more static variables

In the commit referenced below, a '__thread' attribute
was added to numerous static variables to protect them
from concurrent access conflicts with multi-threading.

Unfortunately, that patch did not go quite far enough.

So, this commit adds a few more '__thread' qualifiers.

Reference(s):
commit 23cfb71366

Signed-off-by: Jim Warner <james.warner@comcast.net>

proc/pwcache.c

@@ -38,7 +38,7 @@
 
 static char ERRname[] = "?";
 
-static struct pwbuf {
+static __thread struct pwbuf {
     struct pwbuf *next;
     uid_t uid;
     char name[P_G_SZ];
@@ -67,7 +67,7 @@ char *pwcache_get_user(uid_t uid) {
     return((*p)->name);
 }
 
-static struct grpbuf {
+static __thread struct grpbuf {
     struct grpbuf *next;
     gid_t gid;
     char name[P_G_SZ];