Allow existing VACUUM options to take a Boolean argument.

Allow existing VACUUM options to take a Boolean argument.

This makes VACUUM work more like EXPLAIN already does without changing
the meaning of any commands that already work.  It is intended to
facilitate the addition of future VACUUM options that may take
non-Boolean parameters or that default to false.

Masahiko Sawada, reviewed by me.

Discussion: http://postgr.es/m/CA+TgmobpYrXr5sUaEe_T0boabV0DSm=utSOZzwCUNqfLEEm8Mw@mail.gmail.com
Discussion: http://postgr.es/m/CAD21AoBaFcKBAeL5_++j+Vzir2vBBcF4juW7qH8b3HsQY=Q6+w@mail.gmail.com