Consolidate redundant definitions in s/bsd-common.h.

Consolidate redundant definitions in s/bsd-common.h.
* s/bsd-common.h (BSD4_2, TABDLY, TAB3, HAVE_TERMIOS, NO_TERMIO)
(LIBS_DEBUG, SYSV_SYSTEM_DIR, UNEXEC, NARROWPROTO, BSD_PGRPS)
(LDAV_SYMBOL, KERNEL_FILE): Define (or undefine) here instead of
doing it in all files that include this one.
* s/gnu.h (BSD4_2, TABDLY, TAB3, HAVE_TERMIOS, NO_TERMIO)
(LIBS_DEBUG, SYSV_SYSTEM_DIR, UNEXEC, NARROWPROTO, BSD_PGRPS)
(LDAV_SYMBOL, KERNEL_FILE): Remove.
* s/freebsd.h (BSD4_2, TABDLY, TAB3, HAVE_TERMIOS, NO_TERMIO)
(LIBS_DEBUG, SYSV_SYSTEM_DIR, UNEXEC, NARROWPROTO, BSD_PGRPS)
(LDAV_SYMBOL, KERNEL_FILE): Remove.
* s/netbsd.h (BSD4_2, TABDLY, TAB3, HAVE_TERMIOS, NO_TERMIO)
(LIBS_DEBUG, SYSV_SYSTEM_DIR, UNEXEC, NARROWPROTO, BSD_PGRPS)
(LDAV_SYMBOL, KERNEL_FILE): Remove.