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?
@mikemcquaid Yes I think there is some confusion here and I would like to get clarification.
There are 2 separate diff threads of comments on the same file
I thought the 1st thread was talking about OsxfuseDependency (the diff focused on the top of the file) and the 2nd thread was talking about ConflictsWithFuseForOsx.
However in your latest comment in the 1st thread, is seems like you may have been talking about ConflictsWithFuseForOsx all along?
On that note, your last comment in the 2nd thread said you were deferring to another maintainer for comment...

In general, it seemed like your position was that it was ok to have the binary fuse4osx installed (i.e. in /usr/local) and the brew osxfuse installed simultaneously as long as HOMEBREW_PREFIX!=/usr/local. I'm not privvy to how osxfuse actually works, but from what I know it requires a filesystem driver installed in /Library/Filesystems to function (via post-install steps)  I don't see how it benefits to have the binary fuse4osx 2.7 installed, and a potentially different brew osxfuse version installed **without** it's filesystem driver (not to mention many fuse dependent formula build scripts check /usr/local first, so which is used might be indeterminate).  You suggested this might be like Python, but I'm not sure it's an apples-to-apples comparison in this case b/c the filesystem driver could never be installed by both at the same time.  Unless I missed some other consideration, please let me know...  Thanks