pgsql: Change EDITOR_LINENUMBER_SWITCH to an environment variable

Change EDITOR_LINENUMBER_SWITCH to an environment variable

Also change "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. Move the documentation around a bit to
centralize the editor configuration under environment variables,
rather than repeating bits of it under every backslash command that
invokes an editor.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/79b3ca06d820032ad84446e0a021b56422172d86

Modified Files
--------------
doc/src/sgml/ref/psql-ref.sgml | 103 ++++++++++++++++++++++-----------------
doc/src/sgml/release-9.1.sgml | 2 +-
src/bin/psql/command.c | 19 ++++---
src/bin/psql/settings.h | 2 +
4 files changed, 72 insertions(+), 54 deletions(-)