Rename `silent' mode to `silent-rules' mode.

Rename `silent' mode to `silent-rules' mode.

* automake.in (define_verbose_var, verbose_flag)
(define_verbose_tagvar, handle_options, handle_languages)
(handle_configure, parse_arguments): Rename the `silent' option
to `silent-rules', so it coincides with the `--silent-rules'
command line option; adjust all code and comments.
* lib/Automake/Options.pm (_process_option_list): Likewise.
* doc/automake.texi (Invoking Automake, Libtool Flags, Options):
Likewise.
* NEWS: Adjust.
* tests/dollarvar.test, tests/silent.test, tests/silent2.test,
tests/silent3.test, tests/silent4.test, tests/silent5.test,
tests/silent6.test, tests/silent7.test: Likewise.
Suggestion by Jan Engelhardt.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>