the new page map.
(mmap_record): Change type of base_address_ to caddr_t.
Add map_map_ member. Add several methods to manipulate map_map_.
(mmap_record::alloc_map): New method.
(mmap_record::free_map): Ditto.
(mmap_record::find_empty): Ditto.
(mmap_record::map_map): Ditto.
(mmap_record::unmap_map): Ditto.
(list::add_record): Change return type to `mmap_record *'.
Allocate page map.
(list::match): New method.
(mmap): Partly rewritten to take care for the new page map. Add some
parameter checking.
(munmap): Ditto.