This incorporates:
2020-03-28 Use module 'filename' instead of module 'dosname'
2020-03-28 dosname: Redirect to 'filename'
* lib/at-func.c, lib/canonicalize-lgpl.c, lib/dosname.h:
Copy from Gnulib.
* lib/gnulib.mk.in, m4/gnulib-comp.m4: Regenerate.
* lib/filename.h: New file, copied from Gnulib.
