[lldb] [MainLoop] Add kevent() EINTR handling

Add missing EINTR handling for kevent() calls.  If the call is
interrupted, return from Poll() as if zero events were returned and let
the polling resume on next iteration.  This fixes test flakiness
on NetBSD.

Includes a test case suggested by Pavel Labath on D42206.

Differential Revision: https://reviews.llvm.org/D58230

llvm-svn: 354122