Here's a patch to fix what has been discussed:
* Change EDITOR_LINENUMBER_SWITCH to environment variable.
* I also changed "switch" to "arg" because "switch" is a bit of a
sloppy term.
* So the environment variable is called
PSQL_EDITOR_LINENUMBER_ARG.
* Set "+" as hardcoded default value on Unix (since "vi" is the
hardcoded default editor), so many users won't have to configure
this at all.
* I moved the documentation around a bit to centralize the editor
configuration under environment variables, rather than repeating
bits of it under every backslash command that invoked an editor.
