Remove support for getpeereid()

This function doesn't propvide any added value compared to
getsockopt(...,SO_PEERCRED,...), and is implemented in BSD's
as a wrapper around the latter.

In addition, using SO_PEERCRED instead gives the XRes extension
access to the pid() of the processes of local clients.

Signed-off-by: Matthieu Herrb <matthieu@herrb.eu>
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