Fix URL fragment target

Clarify Homebrew/git install guidance

Properly escapes commas in c flags

* Move Homebrew to the top, common case for new devs
* Note that `brew install rbenv` includes ruby-build by default
* Be clear that `rbenv init` instructions are needed for shell setup
* Be explicit about starting a new shell to pick up PATH/init changes
* Use rbenv-doctor to conclusively demonstrate correct setup
* Separate upgrade instructions for Homebrew vs Git installs

Related: https://github.com/rbenv/ruby-build/pull/1106

Prefer 'set' over 'setenv' for fish shell

Make homebrew installation instructions more clear

Revert back to original quoting style used before previous commit.
Adjust init.bats to reflect changes for successful tests.

I work on a team that has followed the Homebrew installations. More than once we've missed the `rbenv init` instruction and it has caused headaches down the road. This formatting makes it harder to miss.

The setenv function in fish shell has changed dramatically in
75600b6b53
It now conforms to the csh version, which takes at most two arguments.
In this init script, the form
    setenv PATH prepend_something $PATH
had been used, which had too many arguments.
Since setenv isn't a native command in fish, a suitable replacement is
to use the "set -gx" command, which can consume multiple arguments.

Conforms OLD_RBENV_VERSION to RBENV_* convention

Keeping rbenv-controlled variables to RBENV_* "namespace" helps with
discoverability (and tools like rbenv-env) but also consistency and a
very minor degree of safety/isolation from env impact.

Fish shell "." is deprecated in favor of "source"

This ensures that OLD_RBENV_VERSION is never exported. This makes the
implementation a little bit more complex, since more logic needs to be
pushed down into eval'd code.

Help message should not be written to stdout for `sh-` commands

src/configure: Add FreeBSD

The literal tilde in a PATH entry (e.g. `~/.rbenv/shims`) doesn't seem
to be supported by system `which` utility, but *does* seem to be
supported by `command -v` (used in `rbenv-which`) and `type -p`.

Therefore, we must strip away `~/.rbenv/shims` from PATH when looking up
executables for system Ruby, lest we risk infinite loop. We do so by
substituting any occurence of `~` in PATH with the value of `HOME`.

Add a workaround system executable in `/bin`

Update `shobj-conf`

Update the `shobj-conf` script imported from bash to support new OSs.

Properly unset CDPATH

Per [the fish documentation for "source"](file:///usr/local/Cellar/fish/2.2.0/share/doc/fish/commands.html#source) - ". (a single period) is an alias for the source command. The use of . is deprecated in favour of source, and . will be removed in a future version of fish."

Update `rbenv init` instructions

The README details `eval`ing `rbenv init -`, but for some shells (such as fish) there's a difference in what should be run. It turns out that `rbenv init` on its own will print correct instructions, so we should point users to running that command instead.

Updated Mac OSX brew install command

It seems rbenv now comes with ruby-build.  I have not investigated fully, but the previously shown command: `brew install rbenv ruby-build` caused issues on my machine.  After uninstalling both and simply running `brew install rbenv` everything worked fine.

`rbenv shell -` allows you to switch to the previously activated ruby
version.  Similar to `cd -` or `git checkout -`.

This tries to implement `rbenv shell -` as proposed in #854.  However,
adding support seemed to break the "shell change version" test.  I'm not
very good at Bash programming, can someone tell me what is wrong with
what I'm doing?  I'd like to add a bit more functionality to this, but
I'm really just cargo cult programming Bash.

Thank you!

fix tests

Extract hook helper

Use extracted create_hook helper in tests for:

- exec
- hooks
- rehash
- version-name
- version-origin
- which

Spaces in hook path is tested in test/hooks.bats