Only set the fd to NONBLOCK if current_uthread
Fix order of disable_depth with NO_INTERRUPT flag
Add socket and accept calls for syscall interception

Unfortunately, these calls don't have nice wrappers in libc, so we have to
force the application to do the wrapping via -Wl,wrap.  If they don't, the
syscalls won't be intercepted.
No need to save and restore so much on a ctxswitch

Since we always do a cooperatie yield to do a context switch under parlib, we
don't actually have to save / restore every register like the default glibc
get/setcontext calls do. We only have to save the callee saved registers associated
with a function call in the x86_64 function call ABI.

http://www.x86-64.org/documentation/abi.pdf

Additionally, we don't have to save any floating point state, other than the
control registers listed in the ABI.  This is true whether we actually
cooperatively yeild, or we use preemption to yield via parlib's alarm interface.
The signal handler for our preemptive alarms take care of saving and restoring
the proper state for us.

This should speed up contet switching significantly.