Introduce an $MF front-controller for access to executables provided by libs

[ezzatron]

This came up in verbal discussion related to #25, where we were discussing what to call the cross-platform open command.

Having a make variable of $(MF) point to a front-controller for executables would allow us to use $(MF) open <path/url> without worrying about conflicting OS commands. It also means we don't need to mess with PATH, which is how lib-core currently makes its executables available.

It will also be necessary to consider how this feature would support private "extensions" to makefiles.dev that would contain their own libs with executables.