Implement memory mapped files

[hangpark]

Support following two system calls:

mapid_t mmap (int fd, void *addr)
Maps the file open as fd into the process’s virtual address space. The entire file is mapped into consecutive virtual pages starting at addr.

void munmap (mapid_t mapping)
Unmaps the mapping designated by mapping, which must be a mapping ID returned
by a previous call to mmap by the same process that has not yet been unmapped.