Patch for listing runtime option details through server executable (pg_guc)

The attached patch adds the --long-help option to the server executable.
This option displays all the available runtime options for that
particular server version, along with Max, Min and Reset values if
applicable and a description. It also groups the runtime options
together in accordance with the documentation.

--long-help -h provides details, (including):

With no options specified, it will output all available runtime options
in human friendly format, grouped by category and sorted by name.

Usage:
Usage for --long-help option: [-g <group>] [-h] [-H] [-G] [-l]
[string]


General Options:
[string] All options with names that match this string
-g GROUP All options in categories that match GROUP
-G Do not sort output, by group or name

Output Options:
-m Machine friendly format. All fields are tab separated.
-M Same as m, except header with column names is suppressed
-l Prints list of all currently defined groups \ subgroups
-h Prints this help message

The human readable option can be used to help manually configure the
postgresql.conf file, while the machine readable format can be used by
server side utilities to help configure GUC variables. (for example a
GUI, for which this was created)

Thanks,
Aizaz Ahmed

Attachments
-----------
patch file: pg_guc.patch,
new files: src/include/utils/guc_vars.h, /src/include/utils/pg_guc.h,
src/backend/utils/misc/pg_guc.c

To be applied to 7.4 devel.