perf: perform password auth before checking UID.
perf: perform password auth before checking UID.

On FreeBSD DevAuth was showing up as consuming ~10% of the time when doing big
camputs.  It disappears with this change. FreeBSD and Mac exec external
programs for localhost auth, so I imagine they'll both benefit.
On linux, checking the password in memory is (probably) still faster than
reading from /proc (which I'm assuming turns into a roundtrip
userland->kernel->userland which is nice to avoid).

Making the change to UserPass under the assumption the performance improvement
would be similar.  Untested/unprofiled though.

Change-Id: Idb7e888df6e9a36db0be671a44911e018eb7986e

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