Instead of passing large swaths of boolean arguments, define some flags
that can be used in a bitmask.  This makes it easier not only to figure
out what each call site is doing, but also to add some new flags.

The flags are split in two -- one set for index_create directly and
another for constraints.  index_create() itself receives both, and then
passes down the latter to index_constraint_create(), which can also be
called standalone.

Discussion: https://postgr.es/m/20171023151251.j75uoe27gajdjmlm@alvherre.pgsql
Reviewed-by: Simon Riggs