Added per-task curdir. and chdir, pager_read/write, mmap and munmap calls.

They look like what they should look like, but untested.

tasks/mm0/include/syscalls.h

@@ -24,6 +24,8 @@ struct sys_mmap_args {
 int sys_mmap(l4id_t sender, void *start, size_t length, int prot,
 	     int flags, int fd, off_t offset);
 
+int sys_munmap(l4id_t sender, void *vaddr, unsigned long size);
+
 struct sys_shmat_args {
 	l4id_t shmid;
 	const void *shmaddr;