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.

security: drop and regain privileges in all threads

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.

pam_fscrypt is crashing with a segfault in copyIntoSecret() when using
Ctrl-C to interrupt a 'sudo' prompt.  It is dereferencing a NULL pointer
that is supposed point to the PAM_AUTHTOK item.  The problem is that the
Go code assumes pam_get_item() returns a non-success status if the item
is unset, when actually it sets the data pointer to NULL and returns
PAM_SUCCESS.

Fix it by making pam.Handle.GetItem() return an error in that case.

Running "go vet -shadow ./..." finds all places where a variable might
be incorrectly or unnecessarily shadowed. This fixes some of them.

The tests added in this change are trivial, but they make sure that
every package has a non-zero number of tests. This is important for
eventually increasing test coverage.

Now instead of spawning a seperate thread we alternate between changing
the euid and ruid to both find the keyring and link it to the process
keyring. Note that we also ensure that the user keyring is linked into
the root keyring whenever possible.

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

The functions are now changed to (Start|Stop)AsPamUser to indicate that
they handle privilege modification and keyring setup.