Publish GUC flags to custom variables

Hello,

Postgres supports to add custom GUC variables on runtime, but we
cannot use GUC flags in them. This patch adds the flags argument
to DefineCusomXxx() functions. The flags were always 0 until now.

GUC flags are useful for variables with units. Users will be able
to add configuration parameters somothing like memory-size or
time-duration more easily.

I have a plan to use the feature in SQL tracing and analyzing add-on
for postgres. Also, the auto-explain patch suggested in the last
commit-fest could be re-implemented as a plug-in instead of a core-feature
using the custom variable with units and ExecutorRun_hook.

Regards,
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ITAGAKI Takahiro
NTT Open Source Software Center