Add lwkt_sleep() to formalize a shortcut numerous bits of code have been

Add lwkt_sleep() to formalize a shortcut numerous bits of code have been
using for a while, which is to directly deschedule oneself and switch away.
This method of blocking requires a direct lwkt_schedule() call to reschedule
the thread and is primarily used by the message port abstraction.

Change the psignal code to check TDF_SINTR in the thread flags instead
of checking MSGPORTF_WAITING in the thread's private message port.

The lwkt_waitmsg() and lwkt_waitport() functions use the same msgport
backend function (mp_waitport).  Separate the backend into two functions,
mp_waitport and mp_waitmsg, and allow tsleep flags to be passed in instead
of flagging interruptability in the lwkt_msg flags.

Optimize the lwkt_waitmsg() backends - in the fully synchronous critical
path case no critical sections or spinlocks are required at all.