PATCH : code simplification

dlna_dms_init set the VFS default type to "MEMORY"
the calls to dlna_dms_set_vfs_storage_type set VFS storage set DB if requested.
And fall back to MEMORY if it's impossible
PATCH :  change DLNA_DMS_STORAGE__SQL_DB to DLNA_DMS_STORAGE_DB

SQL_DB is appropriate if we use SQL DB. But DB could be NOSQL DB
moves from dms_set_sql_db to dms_db_open

as the storage type, dms_set_sql_db means we use SQL. But it's false if we want to use NOSQL db in the future.
new DLNA status VFSEMPTY. dlna_db_load load DB to build VFS

dlna.h define he new DLNA_ST_VFSEMPTY, to inform that the VFS is not build.
dlna_dms_set_vfs_storage_type try to load the VFS from the DB and return OK
dms_db_load is the new function to load the VFZ from the DB.
dms_db_load follw the db to build the VFS