Add support for process ID and names

Adds the process ID and name of the other side. Useful if you have
multiple clients. Support for OpenBSD could be added in theory since
it has a similar "struct sockpeercred" structure (not sure about
process name though).

After this change, the process ID and name (when available) will be
appended between parentheses:

     0.01: Client (pid 23433 xdpyinfo) -->    4 bytes
             ............REQUEST: ListExtensions
     0.01:                                   332 bytes <-- X11 Server (pid 8290 Xorg)
                                             ..............REPLY: ListExtensions
                                                           names: (29)

Signed-off-by: Peter Wu <peter@lekensteyn.nl>
Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>