negotiate_kerberos_auth: MEMORY keytab and replay cache support

negotiate_kerberos_auth: MEMORY keytab and replay cache support

1) Checks for MEMORY: keytab support and reads the keytab from disk into
   MEMORY to improve performance (i.e. read keytab only at startup and
   nerver again)

2) Add option for replay cache type. Allows to set replay cache to none
   to improve performance ( may reduce security a bit )

3) Add option for replay cache directory.  If /var/tmp is not the best
   location you can choose a different location.