build: fix link failure on cygwin

Cygwin 1.5 has a broken sleep, and the gnulib tests dragged in
rpl_sleep which then caused a link failure because it wasn't in
libcoreutils.a.  We could solve it by using the gnulib sleep module.
However, sleep and usleep may interact poorly with SIGALRM, and they
have less granularity; so it is better to adopt a policy that if we
must sleep, prefer xnanosleep.

* src/sort.c (pipe_fork): Use xnanosleep, to avoid the need for
rpl_sleep on cygwin, and to reduce granularity.
(MAX_FORK_TRIES_COMPRESS, MAX_FORK_TRIES_DECOMPRESS): Increase,
to account for reduction in granularity.
* src/tail.c (tail_file): Use xnanosleep in debug code.
* cfg.mk (sc_prohibit_sleep): New rule.

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