PCP Updates: Check user credentials for pmlogger control connections via AF_UNIX

[Dave Brolley <brolley@xxxxxxxxxx>]

This change implements a simple user credential check for pmlogger 
control connections via the AF_UNIX socket. The implementation 
(hopefully!) as discussed in
http://oss.sgi.com/pipermail/pcp/2014-March/004537.html

which is to initially accept connections from root (uid==0) and from the 
same user as the one running pmlogger. There is a bit of a grey area 
with the latter, as pmlogger is a setuid program which sometimes sets 
its effective user to be the 'pcp' user. In this initial implementation, 
I allowed connections from users with the same uid and euid as pmlogger.
As always the code is on the brolley/dev branch of pcpfans.

Dave
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commit f06a7f06b614feefea14f27b2723ed8606de9e8f
Author: Dave Brolley <brolley@xxxxxxxxxx>
Date:   Tue Mar 18 11:41:57 2014 -0400

    For pmlogger control connections via AF_UNIX, check the connector's 
credentials.
    This initial implementation allows connections by root and
    by the same user as the one running pmlogger.