db-prefix, etc.

Another patch to fix some typos:

filemanager_add_file:

• phpdoc: order of params didn't match order in code
  changed order, added param filename, added return-tag that I think is somewhat correct
• db_next_id: I believe this should be enclosed in '{/}' to use db-prefix
  I have changed this from "file_fid" to "{file}_fid" (most backwardscompatible?) but think maybe "{filemanager}" would be more correct. (If this is supposed to be the same sequence used by upload-module it should be "{files}_fid" instead)
  Note: I am guessing this might affect sites using db-prefix since they will now get a new sequence, but the module should not be working for them anyway since there is another place not using '{/}'
• file -> {file}

filemanager_transfer:

• phpdoc: added param working

filemanager_create_directory:

• phpdoc: added param directory

filemanager_remove_directory:

• phpdoc: added param directory