Ensure that the argument of shmdt(2) is declared "void *".

Ensure that the argument of shmdt(2) is declared "void *".

Our gcc-on-Solaris buildfarm members emit "incompatible pointer type"
warnings in places where it's not.  This is a bit odd, since AFAICT
Solaris follows the POSIX spec in declaring shmdt's argument as
"const void *", and you'd think any pointer argument would satisfy that.
But whatever.  Part of a general push to remove off-the-beaten-track
warnings where we can easily do so.