* m17n.h: Move database related declarations to m17n-core.h

* m17n.h: Move database related declarations to m17n-core.h
(Mharset): Don't extern it.

* m17n-flt.c: Include m17n-core.h instead of m17n.h.
(Mexist): New variable.
(enum FontLayoutCmdRuleSrcType): New enum SRC_EXIST.
(FontLayoutCmdRule): New member src.exist.
(parse_otf_command): Handle gsub_count and gpos_count separately.
(load_otf_command): Adjusted for the change of MFLTOtfSpec.
(load_command): Handle the command "exist".
(free_flt_command): Adjusted for the change of MFLTOtfSpec.
(run_rule): Handle the case SRC_EXIST.
(run_otf): Adjusted for the change of MFLTOtfSpec.
(run_command): Check the range of FROM.
(check_otf_spec): Delete it.
(m17n_init_flt): Call m17n_init_core instead of m17n_init.
Initialize Mexist.
(m17n_fini_flt): Call m17n_fini_core instead of m17n_fiini.
(mflt_find): Call font->check_otf instead of check_otf_spec.
(mflt_run): Keep the metrics in 26.6 fixed point.

* database.c: Include m17n-core.h instead of m17n.h.
(load_chartable): Use mtext__from_data.
(load_charset): Moved to charset.c
(load_database): Call mdatabase__load_charset_func instead of
load_charset.
(mdatabase__load_charset_func): New variable.
(mdatabase__init): Initialize mdatabase__load_charset_func and
Mcharset.  Don't set mdatabase__finder and mdatabase__loader.
(mdatabase__save): Use fwrite, not mconv_encode_stream.
(Mcharset): Declare it here.