• Add explicit conflict-checking to the package tools. Packages can register a list of other packages with which they conflict (via the -C option to pkg_create), and they will refuse to install (unless -f is specified) if one of the listed packages is already present.

• Update documentation for the new feature as well as fleshing out some existing documentation.

• Bump PKG_INSTALL_VERSION so this feature can be tested for.

Submitted by: Sergey Matveychuk <sem@ciam.ru>
PR: bin/47145
MFC after: 2 weeks