Add support for disabling db for verification

Provide a mechanism for a physically present end user to disable the use
of db when doing signature verification.  This is handled by the OS passing
down a variable that contains a UINT32 and a SHA256 hash.  If this variable
is present, MokManager prompts the user to choose whether to enable or
disable the use of db for verification purposes (depending on the value of
the UINT32).  They are then asked to type the passphrase that matches the
hash.  This then saves a boot services variable which is checked by shim,
and if set will cause shim to not use db for verification purposes.  If
db is to be ignored, shim will export a runtime variable called
'MokIgnoreDB' for the OS to query at runtime.

Signed-off-by: Josh Boyer <jwboyer@fedoraproject.org>