The bitmap module already has a simple allocation helper, which returns
an uninitialized, dynamically allocated bitmap of a given size.

This extends this with bitmap_alloc[01]() which allocate bitmaps and
initialize them to all zero or all one.

It also adds bitmap_realloc[01]() which reallocate an existing bitmap,
preserving existing bits, and setting any new bits to all zero or all one.
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>