gssd: Fix handling of the gssname=<name> NFS mount option

If an NFS mount using "sec=krb5[ip],gssname=<name>" is
done, the gssd daemon fails. There is a long delay
(several seconds) in the gss_acquire_cred() call and then
it returns success, but the credentials returned are
junk.

I have no idea how long this has been broken, due to some
change in the Heimdal gssapi library call, but I suspect
it has been quite some time.

Anyhow, it turns out that replacing the "desired_name"
argument with GSS_C_NO_NAME fixes the problem.
Replacing the argument should not be a problem, since the
TGT for the host based initiator credential in the default
keytab file should be the only TGT in the gssd'd credential
cache (which is not the one for uid 0).

I will try and determine if FreeBSD13 and/or FreeBSD12
needs this same fix and will MFC if they need the fix.

This problem only affected Kerberized NFS mounts when the
"gssname" mount option was used. Other Kerberized NFS
mount cases already used GSS_C_NO_NAME and work ok.
A workaround if you do not have this patch is to do a
"kinit -k host/FQDN" as root on the machine, followed by
the Kerberized NFS mount without the "gssname" mount
option.

MFC after: 1 month