libjail: fix handling of allow.mount.fusefs in jailparam_init

fusefs is inconsistently named. The kernel module is named "fuse", but the
mount helper is named "mount_fusefs" and the jail(8) parameter is named
"allow.mount.fusefs". Special case it in libjail.

Reviewed by: jamie
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D17929