Fix linkage of grantpt(3).

Also clean up the implementation of all the pty functions, add tests,
and fix the stub implementations of ttyname(3) and ttyname_r(3).

Bug: https://code.google.com/p/android/issues/detail?id=58888

(cherry picked from commit 4916706c)

Change-Id: I5cb7a1c17b156456e4c4818e65f256eb8d045424