Make munmap() actually work. (#198)

Make munmap() actually work. (#198)

Before this commit, he header of a mapped area, `struct map`, was
defined as follows:

    struct map {
        int prot;
        int flags;
        off_t offset;
        size_t length;
        char body[];
    };

Because the size and alignment of an `off_t` is 8 bytes, the entire
structure was padded to 24 bytes. However, the offset of `body` into
`struct map` was only 20 bytes. Therefore the code in mmap() and
munmap() did not agree on the offset from header to body.

This commit changes mmap() to skip the entire header, which is what
munmap() expects and what the size calculation uses.