Moved lfs_mlist_isopen checks into the API wrappers

This indirectly solves an issue with lfs_file_rawclose asserting
when lfs_file_opencfg errors since appending to the mlist occurs
after open. It also may speed up some of the internal operations such as
the lfs_file_write used to resolve unflushed data.

The idea behind adopting mlist over flags is that realistically it's
unlikely for the user to open a significant number of files (enough for
big O to kick in). That being said, moving the mlist asserts into the
API wrappers does protect some of the internal operations from scaling
based on the number of open files.