When CheckSigningTable is set to no, the keys in KeyTable are no
longer verified when config is loaded. This helps with large
databases. This commit only adds support for USE_ODBX.

when comparing if (conf->conf_checksigningtable == TRUE)
and CheckSigningTable no in the config, no acts as TRUE.
Therefore, it is better to use != FALSE.

As advised by futatuki, specify SigningTable instead of the
"database" in the description.

Allow the use of -C to disable CheckSigningTable (or set to no).

Allow disabling of CheckSigningTable for other databases such as
LDAP. This option disables the walking of SigningTable to look
for missing keys in KeyTable when the config gets loaded.

As requested by futatuki, I will use -g for CheckSigningTable
and reserve -C for future option to check the database tables.