IPC: move utility functions to unix.c

Signed-off-by: Angus Salkeld <asalkeld@redhat.com>
IPC: seperate ipc_us.c into 2 files

Seperate into a setup file and a socket backend file, it was getting messy
and confusing. Also preparing for using DGRAM sockets.

This should not result in any logical changes.

Signed-off-by: Angus Salkeld <asalkeld@redhat.com>