datefn: mod_date(): Add missing return()

148da378-00c0-11e4-b280-c80aa9e67bbd
datefn.t: Remove directory from datefn-files/dir.tar.gz

Yes, .tar files should contain a top directory, but want to get rid of
that chdir() stuff. Also rename dir.tar.gz to file.tar.gz .

bb67cdf2-00c2-11e4-b3f2-c80aa9e67bbd
datefn.t: Add missing diag() for --force

47c2b896-00c5-11e4-9129-c80aa9e67bbd
datefn: Add -s/--skew option + add tests

Also had to update the $Seconds regexp in sec_to_string() to allow wacky
things like timestamps before 1970 in case a huge negative integer is
specified.

c8fe3664-00c6-11e4-991e-c80aa9e67bbd
mc: Run mc(1) inside sess(1) unless $HISTFILE = "/dev/null"

df66173e-00ce-11e4-88fc-c80aa9e67bbd