Linux: unlockpt needs to fail with EINVAL, not ENOTTY (bug 26053)

The EINVAL error code is mandated by POSIX and documented in the
manual.  Also clean up the unlockpt implementation a bit, assuming
that TIOCSPTLCK is always defined.

Enhance login/tst-grantpt to cover unlockpt corner cases.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>