Remove checks for SHORT_FNAME and USE_LONG_FNAME.

They were only defined for MSDOS which is now unsupported.
Don't retry to make the tempname in write_viminfo() with shortname=1

ref #572
Remove the code related to the shortname option
Remove the always-FALSE shortname argument from buf_modname()
Rename buf_modname to modname as they've become the same