refactor: in Automake::Options (no semantic change)

refactor: in Automake::Options (no semantic change)

* lib/Automake/Options.pm: Prefer leading spaces to leading tabs
throughout.  Minor whitespace and comment changes.
(_process_option_list): Simple refactoring to make the code more
pleasant to read and easier to modify in the future.  This
refactoring also reduces code duplication, with the help of ...
(_option_must_be_from_configure, _is_valid_easy_option): ... these
new internal subroutines.
* tests/tar3.test: Enhance.
* tests/silent-amopts.test: New, checks that automake complains if
the 'silent-rules' option is used in AUTOMAKE_OPTIONS.
* tests/list-of-tests.mk: Add it.