Less awkward file.check_hash(...)

Named parameters on the command line are given on the form "key=val".
This makes things awkward when Salt command line needs to be written on
the form:

    salt file.check_hash /etc/myfile hash=md5=abcabcabc

I'd much prefer

    salt file.check_hash /etc/myfile md5:absabcabc

192d9c4439903b9ae940e76cbf3fd2e4accf02a3 makes sure this change is backward compatible to support the old syntax.