idmapd: allow non-ASCII characters (UTF-8) in NFSv4 domain name

The validateascii() check in imconv() maps NFSv4 domain names with
non-ASCII characters to 'nobody'. In setups where Active directory
or LDAP is used this causes names with UTF-8 characters to being
mapped to 'nobody' because of this check.

As Bruce Fields puts it:

"idmapd doesn't seem like the right place to enforce restrictions on
names.  Once the system has allowed a name it's too late to be
complaining about it here."

Replace the validateascii() call in imconv() with a check for
null-termination just to be extra-careful and remove the validateascii()
function itself as the only user of that function is being
removed by this patch.

Acked-by: J. Bruce Fields <bfields@fieldses.org>
Signed-off-by: Suresh Jayaraman <sjayaraman@suse.com>
Signed-off-by: Steve Dickson <steved@redhat.com>