sql: get results of PRAGMA statement as result set

Currently box.sql.execute ('PRAGMA') returns nothing, but prints
list of pragmas and their statuses to stdout. Such strategy is
considered to be wrong since output of this command would be
unavailable for users who redirect stdout, use net box connection
etc. This patch makes the command to return result as the rest of
SQL commands. The result contains only FLAG-type pragmas and their
statuses.