Make sure asterisk builds on OpenBSD

OpenBSD defines SO_PEERCRED, but it returns a 'struct sockpeercred', not
'struct ucred', which causes compilation of main/asterisk.c to fail in
read_credentials().  This allows configure to check for sockpeercred and
asterisk to deal with it properly.

(closes issue ASTERISK-18929)
Reported-by: Barry Miller
Patch-by: Barry Miller
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Merged revisions 350730 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 350731 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@350732 65c4cc65-6c06-0410-ace0-fbb531ad65f3