Add support for AT_REMOVEDIR in unlinkat syscall

[mdboom]

It turns out the CPython regression test suite calls unlinkat with the AT_REMOVEDIR flag a lot.
It's in POSIX 2008: http://pubs.opengroup.org/onlinepubs/9699919799.2008edition/

    AT_REMOVEDIR
        Remove the directory entry specified by fd and path as a directory, not a normal file.

    If unlinkat() is passed the special value AT_FDCWD in the fd parameter, the current working directory is used and the behavior shall be identical to a call to unlink() or rmdir() respectively, depending on whether or not the AT_REMOVEDIR bit is set in flag.

[kripken]

lgtm, thanks! Please just add a test for this. Should probably be in other (we should move more filesystem tests there eventually). See e.g. test_init_file_at_offset there (would be nice to put the test source under tests/fs/ though, which I see that existing test doesn't do).

[mdboom]

Test added.

[kripken]

Ah, sorry, how we do tests is somewhat confusing. self.emcc_args and do_run_from_file are just for the core test suite.

I opened #6473 now to add a simple helper function for running tests in the other suite, with an example there which is how the test here could be done.

Otherwise, lgtm, thanks!

[mdboom]

Cool. I've updated this to use the new test convention.

[kripken]

Great, thanks :)