nfsd: add proc file listing kernel's gss_krb5 enctypes

Add a new proc file which lists the encryption types supported
by the kernel's gss_krb5 code.

Newer MIT Kerberos libraries support the assertion of acceptor
subkeys.  This enctype information allows user-land (svcgssd)
to request that the Kerberos libraries limit the encryption
types that it uses when generating the subkeys.

Signed-off-by: Kevin Coffman <kwc@citi.umich.edu>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>