Main: Added main_teardown() and main_destroy()

Main: Added main_teardown() and main_destroy()

There are now two possibilities how to get rid of a main loop
context. Either you call main_delete(), which is gentle and fails
whenever there are any active files/hooks/etc. Or you call main_destroy(),
which forcibly removes all such objects; it also closes all files.

main_teardown() just calls main_destroy() on the current context,
just as main_cleanup() calls main_delete().

Also, file_close_all() is no longer available -- the only known use
cases are after fork(), but closing files is no longer sufficient
there. All calls to file_close_all() should be therefore replaced
by main_teardown().