capsicum: rename cap_getrights to cap_rights_get

For consistency with FreeBSD10.x Capsicum implementation, call
this syscall cap_rights_get() rather than cap_getrights().
capsicum: fail attempts to widen rights

For consistency with FreeBSD behaviour, explicitly fail attempts
to widen the rights of a capability rather than silently using
a subset.
capsicum: fix capability installation bugs

Fold capsicum_install_fd() into do_sys_cap_new(), which fixes
two long-standing bugs:
 - The struct file* passed to capsicum_install_fd() was outside
   of its read-side critical section (i.e. after rcu_read_unlock())
 - Both capsicum_install_fd() and do_sys_cap_new() were incrementing
   the refcount on the underlying file.

Also export sys_cap_new instead of capsicum_install_fd.