Reduce duplication in dynamic syscall registration code.

Remove the unused syscall_(de)register() functions in favor of the
better documented and easier to use syscall_helper_(un)register(9)
functions.

The default and freebsd32 versions differed in which array of struct
sysents they used and a few missing updates to the 32-bit code as
features were added to the main code.

Reviewed by: cem
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D14337