add pg_strom.enabled_global for test/benchmark purpose
a cosmetic fix
bugfix: GpuPreAgg was incorrectly injected if Agg(AGG_SORTED) uses IndexOnlyScan

It was reported as incorrect group generation because Agg assumed
sorted input stream, but GpuPreAgg breaks this assumption.
This fix allows to treat three cases.
 1. Agg(AGG_HASHED or AGG_PLAIN) --> Inject GpuPreAgg simply
 2. Agg(AGG_SORTED) with Sort --> Inject GpuPreAgg below the Sort
 3. Agg(AGG_SORTED) without Sort --> Replace Agg strategy to AGG_HASHED
    and inject GpuPreAgg, if work_mem configuration allowed.
Merge branch 'master' into numeric

Conflicts:
	gpupreagg.c