memfd-util: introduce memfd_new_full() helper

memfd-util: introduce memfd_new_full() helper

This is just like memfd_new(), but allows fine grained control of the
sealing flags.

This switches over all uses of memfd_new() where we actually want
sealing to use memfd_new_full().

This then allows use to use memfd_new() for two further calls, where we
previously used the more lowlevel memfd_create_wrapper().