Replaced old unit tests for errno with libuv error codes UV_ENOENT
and UV_EEXIST (for os_open and os_getperms).

Added libuv include path to test/includes compiler calls - needed
to get hold of libuv headers.

os_file_is_readonly() in its current form is equivalent to
!os_file_is_writable(). This does not appear to be a bug, because Vim's
use of check_file_readonly() (which we changed to os_file_is_readonly())
is equivalent to !os_file_is_writable() in every case.

os_file_is_readonly() also fails this test:

    returns false if the file is non-read, non-write

A more useful form would define behavior under these cases:

  - path is executable (but not writable)
  - path is non-existent
  - path is directory

But there is no reason for os_file_is_readonly() to exist, so remove it.

AFAICT busted does not report pending() invocations without the 2nd
argument.

- Add event loop abstraction module under src/nvim/event. The
  src/nvim/event/loop module replaces src/nvim/os/event
- Remove direct dependency on libuv signal/timer API and use the new abstraction
  instead.
- Replace all references to uv_default_loop() by &loop.uv, a new global variable
  that wraps libuv main event loop but allows the event loop functions to be
  reused in other contexts.