smbd:smb2: separate between encryption required and enc desired

smbd:smb2: separate between encryption required and enc desired

this means we:
- accept unencrypted requests if encryption only desired
  and not required,
- but we always send encrypted responses in the desired
  case, not only when the request was encrypted.

For this purpose, the do_encryption in the request
structure is separated into was_encrypted and do_encryption.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11372

Signed-off-by: Michael Adam <obnox@samba.org>
Reviewed-by: Guenther Deschner <gd@samba.org>