Patch: back-passwd needs pwent mutex (ITS#1794)

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Written by Hallvard B. Furuseth and placed into the public domain.
This software is not subject to any license of the University of Oslo.
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back-passwd uses getpwent() and setpwfile(), which use static data.
It needs a mutex to make sure these operations can complete without
interference from another back-passwd call.  Here is a patch.

Hallvard B. Furuseth <h.b.furuseth@usit.uio.no>, May 2002.