There are places in the kernel that would like to have access
to `pgid` credentials in certain circumstances.
I haven't found any use cases for `sid` yet, but `sid` and `pgid` are
both changed with `sys$setpgid`, so it seemed sensical to add it.
In Linux, `man 7 credentials` also mentions both the session id and
process group id, so this isn't unprecedented.
Instead of getting credentials from Process::current(), we now require
that they be provided as input to the various VFS functions.
This ensures that an atomic set of credentials is used throughout an
entire VFS operation.
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.
