Isolation: Add a new nxt_cred_t type

This is a generic type to represent a uid_t/gid_t on Linux when user
namespaces are in use.

Technically this only needs to be an unsigned int, but we make it an
int64_t so we can make use of the existing NXT_CONF_MAP_INT64 type.

This will be used in subsequent commits.

Reviewed-by: Zhidao Hong <z.hong@f5.com>
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

src/nxt_clone.h

@@ -9,6 +9,8 @@
 
 #if (NXT_HAVE_CLONE_NEWUSER)
 
+typedef int64_t                 nxt_cred_t;
+
 typedef struct {
     nxt_int_t                   container;
     nxt_int_t                   host;