This way it's clearer to users that an argument is expected. It's
likely that this commit does not catch all such cases, but it should
do it for most of the affected commands I have installed.
This implementation uses -w which is available only in psql >= 8.4,
but the net effect with earlier versions should be the same as when
the completions were disabled.
Turn it on dynamically when needed instead; see doc/styleguide.txt for
a longer explanation. This fixes many non-filename completions which
had been previously more or less broken due to unwanted
escape-as-filenames behavior.
