Better error message for `rbenv shell`

Shell integration is not enabled by default. This means that, from all the
commands from `rbenv commands`, only "shell" won't work right away.

Replace "no such command" with a more descriptive message that points to
`rbenv init` instead.

Fix some issues reported by shellcheck

Fix fish subcommand completion

Fixed `rbenv init -` output to work w/ no args and bash's `set -u`.

partly resolve #759

test/which.bats: do not export PATH

rbenv-version-file: ensure that the version file is a file

Fixed support for commas for all environment variables in configure script

Otherwise `sed` might not be found later in bats/libexec/bats-exec-test,
in case `/usr/bin` gets removed.

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

Forwarded from https://github.com/pyenv/pyenv/pull/606.

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.

This prevents CDPATH from becoming an issue.

Closes #866.

- some versions of bash (e.g. 4.3.11) complain about clobbering
  /dev/null, use bash's explicit >| operator to ignore noclober
- not sure if the original #759 report has the same cause
- https://github.com/rbenv/rbenv/issues/759#issuecomment-289326891

This allows subcommand style plugins to properly autocomplete.
Existing commands are not affected. 

Example, say you have support for `rbenv foo bar --flag`, then
this allows the last `--flag` argument to be properly completed.

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.