bool: convert extern variable in mbyte.h

bool: convert extern variable in mbyte.h

* Charset_is_utf8
* mutt_hcache_restore()
* mutt_set_charset()

Charset_is_utf8 affects `convert` in mutt_set_charset() and mutt_hcache_restore()
-> affects several static functions in hcache/hcache.c

* dump_address()
* dump_body()
* dump_buffer()
* dump_char()
* dump_char_size()
* dump_envelope()
* dump_parameter()
* dump_stailq()

* restore_address()
* restore_body()
* restore_buffer()
* restore_char()
* restore_envelope()
* restore_parameter()
* restore_stailq()