New module 'tcgetsid'.

New module 'tcgetsid'.

* lib/tcgetsid.c: New file.
* m4/tcgetsid.m4: New file.
* modules/tcgetsid: New file.
* modules/termios (Depends-on): Add c++defs, warn-on-use.
(Makefile.am): Ensure c++defs.h, warn-on-use.h get included. Substitute
GNULIB_TCGETSID, HAVE_TCGETSID.
* lib/termios.in.h: Include <sys/types.h>.
(tcgetsid): New declaration.
* m4/termios_h.m4 (gl_TERMIOS_H): Check whether tcgetsid is declared.
(gl_TERMIOS_H_DEFAULTS): Initialize GNULIB_TCGETSID, HAVE_TCGETSID.
* doc/posix-functions/tcgetsid.texi: Mention the new module.
* tests/test-termios-c++.cc: Check GNULIB_NAMESPACE::tcgetsid.