Allow IDENT authentication on local connections (Linux only)

Hello,

the attached patch against 7.1.2 allows to use identd-like authentication
on unix domain sockets.

This is done by getting the uid of the connecting process via
getsockopt(... SO_PEERCRED ...) which to my knowledge is Linux-specific,
but I may be wrong.

The patch does not contain a configure macro to enable the feature, add
    #define HAVE_SO_PEERCRED 1
to include/config.h before building to activate it.

I have been using this feature since 7.0.1 and would be grateful if
something like this could be integrated.

Regards,
Helge