[bug-mailutils] opens numeric named files if MAIL is null

[jidanni]

I discovered with mail (GNU Mailutils 1.2) if one does as an experiment
$ cd; MAIL= mail
and one has a file with a name like 211 there, mail will open that
thinking it is your mailbox. Strace shows
open("/home/jidanni//211", O_RDWR|O_LARGEFILE) = 4

Apparently it looks for files that match
ls|grep -v '[^0-9]'
and maybe others.