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)