Fix `clang -Wcast-qual` issues

Fix `clang -Wcast-qual` issues

Remove unnecessary `char*` casting for arguments passed to `cget*(3)`, and
deconst `_PATH_PRINTCAP` before passing it to `cget*` via the `printcapdb`
variable.

This unblocks ^/projects/runtime-coverage-v2 from building cleanly on
universe13a.freebsd.org. I suspect the issue was introduced through some
changes to `bsd.*.mk` inclusion on the branch, which I will continue to
investigate/isolate.

MFC after:	1 week
Tested with:	clang 8 (arm64)