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).
Document the config module in manpages

This adds the manual pages: shtk_config(3), shtk_config_get(3),
shtk_config_get_bool(3), shtk_config_get_default(3), shtk_config_has(3),
shtk_config_include(3), shtk_config_init(3), shtk_config_load(3),
shtk_config_override(3), shtk_config_run_hook(3), shtk_config_set(3) and
shtk_config_unset(3).
Document the cvs module in manpages

This adds the manual pages: shtk_cvs(3), shtk_cvs_checkout(3),
shtk_cvs_fetch(3) and shtk_cvs_update(3).
Document the list module in manpages

This adds the manual pages: shtk_list(3), shtk_list_contains(3) and
shtk_list_filter(3).
Document the process module in manpages

This adds the manual pages: shtk_process(3) and shtk_process_run(3).
Document the version module in manpages

This adds the manual pages: shtk_version(3), shtk_version_at_least(3),
shtk_version_at_most(3) and shtk_version_is(3).