timeout: don’t sleep less than requested

timeout: don’t sleep less than requested

Also, change sleep and tail to not sleep less than requested.
* bootstrap.conf (gnulib_modules): Add dtimespec-bound.
* gl/lib/dtimespec-bound.c, gl/lib/dtimespec-bound.h:
* gl/modules/dtimespec-bound: New files.
* src/sleep.c, src/tail.c, src/timeout.c: Include dtimespec-bound.h.
* src/sleep.c, src/tail.c: Don’t include xstrtod.h.
* src/sleep.c (apply_suffix, main):
* src/tail.c (parse_options):
* src/timeout.c (apply_time_suffix):
Don’t sleep less than the true number of seconds.
* src/timeout.c: Don’t include ctype.h.
(is_negative): Remove; no longer needed.
(parse_duration): Use a slightly looser bound on the timeout, one
that doesn’t need -lm on GNU/Linux.  Clear errno before calling
cl_strtod.