Allow user and group specified from cmdline; and add unix socket utility functions

Hi,

I have all the pfixtools stuff done for net string protocol so it can support postfix socketmap lookup table (which supports unix sockets as well as TCP)

This is the first part, for libcommon, which is quite straight forward. It just add a utility function to open unix socket, and also the user/group specification. It's up to the binary to make use of the user/group in much the same way it's up to the binary to make use of the pidfile option.

Thanks.