Fix Solaris problems with ttyname_r.

Fix Solaris problems with ttyname_r.

* m4/gnupg-ttyname.m4: New.  Based on ttyname_r from gnulib.
* src/ttyname_r.c (_gpgme_ttyname_r): Rename from ttyname_r.
Implement hacks required for Solaris and possible other non-fully
Posix systems.
* src/util.h: Include unistd.h.  Redefine ttyname_r depending on
REPLACE_TTYNAME_R and put it into the gpgme name space.
--

Unfortunately we cant not use the ttyname_r replacement from gnulib
because we want to keep GPGME LGPLv2+.