Use the single-file implementation of MIO from universal-ctags

Use the single-file implementation of MIO from universal-ctags

The version of MIO corresponds to commit 509a47dbc in universal-ctags
which contains just minimal changes mostly related to changing MIO from
a library into a single ctags source file:

- replaced the glib types with ordinary C types
- removed the "virtual" calls and replaced them with simple if/else
- made the implementation in a single file
- reformatted the library to more or less match universal-ctags style
- removed the MIO_FORCE_ANSI ifdef as it included some glib file and
  we don't really need it
- added mio_flush() - of course makes sense just for the file backend
  (calls fflush())
- made mio_free() return error code from fclose()
- changed mio_new_fp() to return NULL when the passed FILE is NULL
  (simplifies logic at one place in ctags and makes sense IMO)