Unix sockets connection authentication - patch

[apologies if this appears twice; I thought I had sent it but it hasn't
appeared anywhere]
The attached patch implements a method of connection authentication for
Unix sockets that support SCM_CREDENTIALS.  This includes Linux kernels
2.2 and 2.4 at least; I don't know what other implementations support
it.

Since it is not universally supported, I have included a configure test.
autoconf needs to be run after installing the patch.

This patch provides a new authentication method "peer" for use with
"local" connections; otherwise it works exactly like the "ident" method.

Please consider including this in PostgreSQL.

Oliver Elphick                                Oliver.Elphick@lfix.co.uk
Isle of Wight                              http://www.lfix.co.uk/oliver
PGP: 1024R/32B8FAA1: 97 EA 1D 47 72 3F 28 47  6B 7E 39 CC 56 E4 C1 47
GPG: 1024D/3E1D0C1C: CA12 09E0 E8D5 8870 5839  932A 614D 4C34 3E1D 0C1C
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      that weep."            Romans 12:15