Unify the “--iallocator” option

This patch also changes the gnt-debug's “--algorithm” into
IALLOCATOR_OPT, since it was the only non-standard use of this (due to
historical reasons). Since it's just a debug option,
backwards-compatibility is not critical here.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>