watch: Add --equexit option

watch: Add --equexit option

This commit adds a new option called `--equexit` which is the opposite
of `--chgexit`. This option makes it possible to exit when the output
of the given command does not change for the given number of cycles.
A download operation could be given as a use-case since `watch` will
exit when the output does not change anymore, in other words, when
the download is completed.

References:
 procps-ng/procps#232

Signed-off-by: Orhun Parmaksız <orhunparmaksiz@gmail.com>