xC: allow passing the cursor position to editors

Add a configuration option to set a custom editor command,
different from EDITOR or VISUAL--those remain as defaults.

Implement substitutions allowing to convey cursor information
to VIM and Emacs (the latter of which is fairly painful to cater to),
and put usage hints in the configuration option's description.

This should make the editing experience a bit more seamless
for users, even though the position is carried over in one way only.

No sophisticated quoting capabilities were deemed necessary,
it is a lot of code already.  The particular syntax is inspired
by .desktop files and systemd.

["/bin/sh", "-c", "vim +$2go \"$1\"", filename, position, line, column]
would be a slightly simpler but cryptic way of implementing this.

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