Consolidate redundant definitions.

Consolidate redundant definitions.
* s/usg5-4.h (sigsetmask, PTY_TTY_NAME_SPRINTF): Do not define,
it's undefined in all files that include this one.
(POSIX_SIGNALS): Define here instead of doing it in all files that
include this one.
* s/irix6-5.h (sigsetmask, PTY_TTY_NAME_SPRINTF): Do not undef.
(POSIX_SIGNALS): Do not define.
* s/sol2-6.h (sigsetmask, PTY_TTY_NAME_SPRINTF): Do not undef.
(POSIX_SIGNALS): Do not define.
* s/unixware.h (sigsetmask, PTY_TTY_NAME_SPRINTF): Do not undef.
(POSIX_SIGNALS): Do not define.