libext2fs: add ext2fs_find_first_set_{block,inode}_bitmap2()

libext2fs: add ext2fs_find_first_set_{block,inode}_bitmap2()

Add functions which try to find the first set block or inode in a
bitmap.  This is useful when trying to allocate a range of blocks
efficiently.

Like the find_first_zero family of functions, provide a generic O(N)
search function which will be used if there is no optimized version
provided by the red-black tree or bitarray functions.

Also, expand the test cases for ext2fs_find_first_zero_*() functions.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>