Patch: Implement surrogate parent for back-shell (ITS#1815)

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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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A surrogate parent is supposed to keep back-shell children from
deadlocking due to resources locked by a threading parent.

Implementation note: The surrogate parent closes all unused file
descriptors, so it logs errors to stderr instead of via Debug() and
uses relloc() instead of ch_realloc().

Also close a file descriptor leak if fork() fails in fork.c.

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