misc/mc{,-nox11}: slightly improve the ports' Makefiles (+)

- Do not globally pollute -I/-L with ${LOCALBASE} when handling
  the NLS option and pass it as the --with-libintl-prefix instead
- Drop no longer supported --with-slang-includes configure switch,
  correct paths are obtained via pkgconf(1) these days
- Set PKGNAMESUFFIX together with CONFLICTS_INSTALL in the master
  so they are always coherent (previously, when the main port is
  built with X11 option unset it would conflict with itself)