Simplify EMACS_TIME-related code.

Simplify EMACS_TIME-related code.

This portability layer is no longer needed, since Emacs has been
using struct timespec as a portability layer for some time.
Merge from gnulib, incorporating:
2013-08-27 timespec: new convenience constants and function
* src/atimer.h, src/buffer.h, src/dispextern.h, src/xgselect.h:
Include <time.h> rather than "systime.h"; that's all that's needed now.
* src/dispnew.c: Include <timespec.h> rather than "systime.h";
that's all that's needed now.
* src/systime.h (EMACS_TIME): Remove.  All uses changed to struct timespec.
(EMACS_TIME_RESOLUTION): Remove.  All uses changed to
TIMESPEC_RESOLUTION.
(LOG10_EMACS_TIME_RESOLUTION): Remove.  All uses changed to
LOG10_TIMESPEC_RESOLUTION.
(EMACS_SECS, emacs_secs_addr): Remove.  All uses changed to tv_sec.
(EMACS_NSECS): Remove.  All uses changed to tv_nsec.
(make_emacs_time): Remove.  All used changed to make_timespec.
(invalid_timespec): Rename from invalid_emacs_time.  All uses changed.
(current_timespec): Rename from current_emacs_time.  All uses changed.
(add_emacs_time): Remove.  All uses changed to timespec_add.
(sub_emacs_time): Remove.  All uses change dot timespec_sub.
(EMACS_TIME_SIGN): Remove.  All uses changed to timespec_sign.
(timespec_valid_p): Rename from EMACS_TIME_VALID_P.  All uses changed.
(EMACS_TIME_FROM_DOUBLE): Remove.  All uses changed to dtotimespec.
(EMACS_TIME_TO_DOUBLE): Remove.  All uses changed to timespectod.
(current_timespec): Rename from current_emacs_time.  All uses changed.
(EMACS_TIME_EQ, EMACS_TIME_LT, EMACS_TIME_LE): Remove.  All uses
changed to timespec_cmp.
* src/xgselect.c: Include <timespec.h>, since our .h files don't.