Add const to input-only char * arguments.

[brooks]

Summary

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.

Depends on D17805

[brooks]

• Rebase

[kib]

Are you going to reformat compat32/syscalls.master ?

[brooks]

In D17812#380637, @kib wrote:

Are you going to reformat compat32/syscalls.master ?

I plan to replace it with improvements to makesyscalls.sh that will let us generate compat32 bits from kern/syscalls.master so I'm not very inclined to do the tedious work of reformatting it. I'm more than half done in CheriBSD where I'm working toward a freebsd64.