Document the base module in manpages

Add manual pages for the functions provided by the base module, which at
the moment is only shtk_import(3).

To support this change, and to allow the discoverability of all function-
specific pages, introduce the new shtk(3) page, mention the former from
shtk(1), and write down some guidelines in the man/README file on how
these pages should be structured..
Document the bool module in manpages

This adds the manual pages: shtk_bool(3) and shtk_bool_check(3).
Document the cleanup module in manpages

This adds the manual pages: shtk_cleanup(3) and shtk_cleanup_register(3).
Document the cli module in manpages

This adds the manual pages: shtk_cli(3), shtk_cli_dirname(3),
shtk_cli_error(3), shtk_cli_info(3), shtk_cli_progname(3),
shtk_cli_usage_error(3) and shtk_cli_warning(3).