sys/syscallsubr.h: assorted tidying

• gc unused struct kevent
• sort kern_frmdirat, kern_funlinkat, kern_kexec_load, and kern_kqueue
• put user_ functions in their own section

Effort: CHERI upstreaming
Reviewed by: kib
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D55309