Removes user_mem_check, fixes syscall bug

This was a good one.  When not MAP_POPULATE-ing anonymous memory,
syscalls where part of the struct was on a page that hadn't been faulted
in yet would fail a user_mem_check.  Silently.  But good thing we set
that global racy variable!  Since run_local_syscall() would just return,
we'd pop back to userspace and the syscall would never complete, because
it never started.  6.5 giraffe-hours btw (* 2).