Invalid (old?) table or database name in logs

Post push patch.

Bug was that a non partitioned table file was not
converted to system_charset, (due to table_name_len was not set).

Also missing DBUG_RETURN.

And Innodb adds quotes after calling the function,
so I added one more mode where explain_filename does not
add quotes. But it still appends the [sub]partition name
as a comment.

Also caught a minor quoting bug, the character '`' was
not quoted in the identifier. (so 'a`b' was quoted as `a`b`
and not `a``b`, this is mulitbyte characters aware.)