Move the mkioctls script to libsysdecode and use it to generate a
sysdecode_ioctlname() function. This function matches the behavior
of the truss variant in that it returns a pointer to a string description
for known ioctls. The caller is responsible for displaying unknown
ioctl requests. For kdump this meant moving the logic to handle unknown
ioctl requests out of the generated function and into an ioctlname()
function in kdump.c instead.

Differential Revision: https://reviews.freebsd.org/D4610