Problem: gettabvar() is not consistent with getwinvar() and getbufvar().
Solution: Return a dict with all variables when the varname is empty.
(Yasuhiro Matsumoto)
Problem: When using ":new ++ff=unix" and "dos" is first in 'fileformats'
then 'ff' is set to "dos" instead of "unix". (Ingo Karkat)
Solution: Create set_file_options() and invoke it from do_ecmd().
Problem: Clumsy to handle the situation that a variable does not exist.
Solution: Add default value to getbufvar() et al. (Shougo Matsushita,
Hirohito Higashi)
