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/CMakeLists.txt

@@ -43,6 +43,7 @@ set(KERNEL_SOURCES
     CMOS.cpp
     CommandLine.cpp
     Coredump.cpp
+    Credentials.cpp
     Devices/AsyncDeviceRequest.cpp
     Devices/Audio/AC97.cpp
     Devices/Audio/Channel.cpp