Reduce duplication in dynamic syscall registration code.

[brooks]

Summary

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 so code was entierly duplicated.

[cem]

Comments below are mostly just observations of differences between the forked versions of these routines. Probably accidental divergence rather than intentional.

sys/kern/kern_syscalls.c

133–134 ↗

(On Diff #39225)

This KASSERT was not present in the old syscall32_register, FWIW.

136 ↗

(On Diff #39225)

This assignment was not present in syscall32_register.

144 ↗

(On Diff #39225)

old_sysent could be const struct sysent *.

152–154 ↗

(On Diff #39225)

This logic (check for static + drain) was not present in the syscall32_deregister.

177 ↗

(On Diff #39225)

flags was always SY_THR_STATIC_KLD in syscall32 version