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).
Remove last remaining docstring-like tags

With the conversion of all documentation to manual pages, we have removed
all Doxygen, docstring-like comments from all modules.  Remove the last
remaining instance of this from bootstrap.subr.
Move shtk.1.in into man

For consistency with the rest of the manual pages, move shtk.1.in
into the new 'man' subdirectory.
Merge branch 'manpages'

This brings in manual pages for all public API functions, with their
starting point being shtk(3).

The docstrings of all public API functions have been removed in favor
of the manual pages.  Only docstrings for private functions remain in
the code.