Don't build ttyemu and run tty I/O test if grantpt is not available.

* configure.ac (TAR_COND_GRANTPT): Define conditional depending
on whether grantpt is available.
* gnulib.modules: Remove grantpt. It relies upon a helper binary
pt_chown which it installs and which is useless in the testsuite.
* tests/Makefile.am [TAR_COND_GRANTPT]: Build ttyemu
* tests/iotty.at: Skip test if ttyemu is not build.
* tests/ttyemu.c (noecho): Fix error message
(main): Use TIOCSCTTY if it is defined.