One-shot expanded output in psql using \G

I frequently find myself in the situation that I want the "\x"
expanded output mode activated just for one query. There's little
wrong with typing "\x" and re-executing the query in that case, but
then I'm always annoyed that the expanded output is still active for
the next query after that.

"\x auto" is not a fix for the problem; I have set up the pager to use
"less -S" (non-wrapping) by default so I can scroll right/left through
the query result instead of having it spread over several terminal
lines.

A workaround is to submit queries using "\x\g\x", but that's ugly,
clutters the output with toggle messages, and will forget that "\x
auto" was set.

Mysql's CLI client is using \G for this purpose, and adding the very
same functionality to psql fits nicely into the set of existing
backslash commands: \g sends the query buffer, \G will do exactly the
same as \g (including parameters), but forces expanded output just for
this query.

The same idea was discussed back in 2008. Back then the outcome was
that "\x auto" was implemented, but I still think that \G is a useful
feature to have on its own, and several people in the thread seem to
have agreed back then.

Patch attached, I'll add it to the next commit fest.

Mit freundlichen Grüßen,
Christoph Berg
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