Add const to input-only char * arguments.

These arguments are mostly paths handled by NAMEI*() macros which already
take const char * arguments.

This change improves the match between syscalls.master and the public
declerations of system calls.

Reviewed by: kib (prior version)
Obtained from: CheriBSD
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D17812