sbr/folder_read.c: Don't malloc() once per message.

Instead of struct msgs having a pointer to a malloc'd array of pointers,
each to a malloc'd struct bvector, 1+N, it now has a pointer to a
malloc'd array of struct bvector;  one malloc for all of them.  This
avoids the large number of calls to malloc() and free() that's linear
with the size of the folder.

But there are some downsides.  In order to step through an array of
struct bvector, code outside of sbr/vector.c needs to know the struct's
size.  The simplest way to do this is to make the struct's definition
public, with a comment that access should be through vector.c.

New functions are needed to initialise the content of an already
allocated bvector, and to finish with its content prior to deallocation.
bvector_create() and bvector_free() now also use these new functions.

Before, it was the array of pointers to bvector that would be realloc'd.
That doesn't work for the array of bvectors as they may contain pointers
to within themselves.  The solution is to malloc a new array and
bvector_copy() the ones to keep across, as folder_realloc() now does.
The other half of its logic that coped with growth at the end of the
array, has been deleted.  Also deleted, is the code to clear the
bvectors before and after the old ones as they start in that state.