termios: make XSI extensions optional.

Mac OS X is a POSIX conformant system, but does not implement
all XSI extensions. Only create constant for XSI extensions if
they are defined by the host system. See:
http://pubs.opengroup.org/onlinepubs/009695399/basedefs/termios.h.html
* lposix.c (CBAUD, EXTA, EXTB, ECHOCTL, ECHOPRT, ECHOKE, FLUSHO)
(PENDIN): Wrap with preprocessor guards.
(DEFECHO, LOBLK, SWTCH, VDISCARD, VDSUSP, VLNEXT, VREPRINT)
(VSTATUS, VWERASE): Additional conditional XSI extensions.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
termios: remove deprecated legacy symbols.

Don't write new code with these, they are not definied by stricly
POSIX conformant hosts. See:
http://pubs.opengroup.org/onlinepubs/009695399/basedefs/termios.h.html
* lposix.c (XCASE, IUCLC, OUCLC): Remove.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>