Kernel: Add Credentials to hold a set of user and group IDs

This patch adds a new object to hold a Process's user credentials:

- UID, EUID, SUID
- GID, EGID, SGID, extra GIDs

Credentials are immutable and child processes initially inherit the
Credentials object from their parent.

Whenever a process changes one or more of its user/group IDs, a new
Credentials object is constructed.

Any code that wants to inspect and act on a set of credentials can now
do so without worrying about data races.

Kernel/Forward.h

@@ -14,6 +14,7 @@ namespace Kernel {
 class BlockDevice;
 class CharacterDevice;
 class Coredump;
+class Credentials;
 class Custody;
 class DevTmpFSDeviceInode;
 class DevTmpFSDirectoryInode;