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
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REL9_1_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/70a488878f97eeec09356054fc161b9f8e914257

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(-)