Improve determineVimFileType: do not require "set", search backwards

 - "set " or "se " is not required with the (newer) modelines
 - prefer later entries in case of multiple settings, by using a new
   strrstr function
Simplify `strrstr` and `strstr`

Thanks @vhda.
Ref: https://github.com/fishman/ctags/pull/125#issuecomment-63169399