nrelease: Support to use binary packages

nrelease: Support to use binary packages

Add the new optional target "binpkgs" to allow to use binary packages,
i.e., installing them with pkg(8), instead of building them from
dports(7).  This can be handy for end users who want to quickly build
a custom image with custom packages.

As pointed out by Sascha Wildner, one port may have multiple binary
packages (e.g., Python flavors) or have a different name that may not be
found by pkg(8).  Therefore, allow to specify the exact binary package
name for a port by setting 'PKG_<port>=<pkg-name>'.

This feature is disabled by default, as suggested by Sascha Wildner,
because the release process should be self-contained as much as possible.

Update the help text and man page accordingly.