MFC r320009,r347075:

r320009 (by sbruno):

Quiesce clang warning while building lpc.

usr.sbin/lpr/lpc/lpc.c

Warning
  passing 'char *[20]' to parameter of type 'const char **' discards
  qualifiers in nested pointer types
  [-Wincompatible-pointer-types-discards-qualifiers]
Fix:
   Explicitly cast the variable "margv" to const char ** only for it's
   use as a parameter to suppress the error

r347075:

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.

Tested with: clang 8 (arm64)