mon: Add mechanism for user to add/clear pool flags.

mon: Add mechanism for user to add/clear pool flags.

Previously, every time we had a new experimental feature, switched with a
pool flag, we needed to add a bunch of boiler plate.  Given that end users
should not be using these features, adding all of this user-visible
behaviour is not desirable.

This adds a single mechanism to specify a flag set by number.  These magic
numbers can be used during development and then either removed, or
promoted to user-friendly flags.

Signed-off-by: Alex Ainscow <aainscow@uk.ibm.com>