rfc: add :osxfuse dependency to support binary or homebrew installs

add :osxfuse symbolic dependency
add ConflictsWithFuseForOsx requirement for osxfuse/fuse4x

**reasons:**
- presently, osxfuse will fail to link the bottle files if the binary install is present (conflicting files), but will not fail a build as a dependency (perhaps this is a bug, but is resolved in this pr)
- if the binary distribution is installed, a user would be forced to uninstall just to use any brew fuse formula (and perform osxfuse postinstall steps)
- some apps check/require the official Fuse for OSX installation. #28298
- PR discussed/suggested in #26211

initial commit includes only Dependency+ConflictsWithFuseForOsx to start a discussion on this feature
i have all osxfuse dependent formulas working locally with both binary/brew fuse installs
can commit some/all formula updates later (so it doesn't swamp brewbot)

**non-fatal issues:**
formula.include references:
afflib: `-I#{Formula["osxfuse"].include}/osxfuse`
- compiles, but probably works as the makefile scans the standard /usr/local/include already
- builds without this directive, only useful if HOMEBREW_PREFIX is not /usr/local

ext2fuse: `-I#{HOMEBREW_PREFIX}/opt/osxfuse/include/osxfuse/fuse`
- fails to compile, quick fix would be to add `-I/usr/local/include/osxfuse/fuse` to cover both cases
- wouldn't expect formula devs to cover both cases long-term

?? live with the quick fix for now?
?- don't see any existing patterns that would yield the system vs formula locations of include/lib
-- (PythonDependency.)which_python is the best poor example I've found
--- ranger.rb calls `PythonDependency.new.which_python`
--- many other formula copy/paste the which_python method (all in science/osgeo4mac taps)

**question:**
does `depends_on :osxfuse` negate the ability to bottle?
...if so, just remove bottle directive from those that have?
Let's use `File.exist?`, as `File.exists?` is deprecated in newer versions of ruby