Rewrite vis-open to be more robust.

Rather than try to loop manually and build up a path, this new version
exec's itself for each new directory scanned. Functional changes
include:

- We `set -e` at the top of the script, so any surprising
  permission-denied errors will automatically cause the script to exit
  with a helpful error message.
- We now support the GNU "--" convention for splitting options from
  filename arguments, just in case somebody happens across a directory
  with a file named "-h" or "-p".
- If launched with a single filename parameter, we automatically return
  it - if somebody writes ":e somepattern*" and it matches exactly one
  file, we might as well open it directly.
- If we select a single filename parameter, we use `realpath` to build
  an absolute path for it - much more reliable than trying to build up
  a path while the user is navigating around.
- If launched with a single directory parameter, we change into that
  directory and re-exec ourselves with ".." and all the files in that
  directory as arguments. This means we don't have to handle selection
  and recursion at the same time.
- Note that if we recurse into a directory, we pass the "-f" parameter
  to suppress auto-selection - otherwise recursing into an empty
  directory would immediately select the ".." entry and pop you back
  out, which would be confusing.
- The new version doesn't bother trying to manipulate `$VIS_MENU_ARGS`
  and sometimes add a prompt to it. Setting no prompt is the same as
  setting the prompt to an empty string, so we can just use an empty
  string as the default value.

One specific use-case that this change cleans up is giving ":e"
a pattern that matches multiple directories (for example, running
":e *e*" in the root of the vis repo). "ls -1" would list the contents
of each directory (without a prefix, so you couldn't select those files)
but would also print the directory names as headings followed by
a colon, so you couldn't usefully select the directory names either.

We get around this by only ever running "ls -1" without any arguments,
so it only scans the current directory.

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