r7827: Add in-memory keytab to Samba4, using the new MEMORY_WILDCARD keytab

r7827: Add in-memory keytab to Samba4, using the new MEMORY_WILDCARD keytab
support in Heimdal.

This removes the 'ext_keytab' step from my Samba4/WinXP client howto.

In doing this work, I realised that the replay cache in Heimdal is
currently a no-op, so I have removed the calls to it, and therefore
the mutex calls from passdb/secrets.c.

This patch also includes a replacement 'magic' mechanism detection,
that does not issue extra error messages from deep inside the GSSAPI
code.

Andrew Bartlett