mount.cifs: make credentials file parameters consistent with mount options

mount.cifs: make credentials file parameters consistent with mount options

This patch makes the mount.cifs credentials file parameters consistent with
the command line parameters to remove ambiguity between the command line
parameter format and the credentials file format. That is, it parses for
both short and long form of the 'username', 'password', and 'domain'
parameters.  This patch is against the current cifs-utils-4.2.

I'm also thinking of adding a second patch that allows for parsing a
"domain/user", "domain%user" and "domain/user%password" formats as allowed
from the command line.

Signed-off-by: Scott Lovenberg <scott.lovenberg@gmail.com>