Switched from sendto/recvfrom to sendmsg/recvmsg for UDS I/O. Replaced

Switched from sendto/recvfrom to sendmsg/recvmsg for UDS I/O. Replaced
inlined sendto/recvfrom hacks with FD_WRITE/READ_METHOD-based code.

A common msghdr-based interface allows us to use the same API for regular
IPC messages and for future messages that pass socket descriptors. While
msghdr allows for complex vector-based scatter/gather I/O, the IPC code
limits complexity by using a single-element I/O vector and a control message
part dedicated to passing descriptors.

Added a temporary hack to block-sleep between IPC message sending attempts so
that we do not use up all the allowed attempts in a short period of time. The
hack will be replace with a non-blocking addEvent-based sleep.