psql :: support for \ev viewname and \sv viewname

Hackers!

I'm proposing to add two new subcommands in psql:
1. \ev viewname - edit view definition with external editor (like \ef for
function)
2. \sv viewname - show view definition (like \sf for function, for
consistency)

What's inside:
1. review-ready implementation of \ev and \sv psql subcommands for editing
and viewing view's definition.
2. psql's doc update with new subcommands description.
3. a bit of extracting common source code parts into separate functions.
4. psql internal help update.
5. tab completion update.

There is one narrow place in this patch: current implementation doesn't
support "create" statement formatting for recursive views.

Any comments? Suggestions?