This change makes sure that, when we set the ruid and euid in order to
get the user keyring linked into the current process keyring, we will
always be able to reverse these changes (using a suid of 0).

This fixes an issue where "su <user>" would result in a system error
when called by an unprivileged user. It also explains exactly how and
why we are making these privilege changes.

This change makes sure after dropping then elevating privileges for a
process, the euid, guid, and groups are all the same as they were
originally. This significantly simplifies the privilege logic.

This fixes CVE-2018-6558, which allowed an unprivleged user to gain
membership in the root group (gid 0) due to the groups not being
properly reset in the process.

After enabling pam_fscrypt for "session" and creating a directory
protected with a login protector, I was no longer able to log in as that
user.  The problem is that the Go runtime is creating threads after
pam_fscrypt drops privileges, but pam_fscrypt is not re-acquiring
privileges on those threads because the Go wrappers for setreuid(),
setregid(), and setgroups() in the "sys/unix" package are using the raw
syscalls which operate on the calling thread only.

This violates glibc's assumption that all threads have the same uids and
gids, causing it to abort() the process when a later module in the PAM
stack (pam_mail in my case) tries to drop privileges using the glibc
functions.

Fix it by dropping and regaining privileges using the glibc functions
rather than the "sys/unix" functions.

This also avoids any possibility that privileges could be changed in a
thread other than the "main" one for pam_fscrypt, since the Go runtime
does not guarantee which OS-level thread runs what.

It would be nice to also exit all Go worker threads before returning
from pam_fscrypt, but the Go runtime doesn't seem to support that.

This was creating an issue becasuse fully dropping privileges required
spawning a goroutine and using rutime.DropOSThread().

The keyring lookup functions no longer read from /proc/keys. Now they
simply spawn a thread, drop privs, and check with GetKeyringID and
KEY_SPEC_USER_KEYRING. See userKeyringID() for more info.

The privileges functions have also been changed. Now the concept of
setting privileges is seperate form the concept of setting up the
keyrings.