Cleaned up libposix mmap()

Now we have proper setting of errno and using pfn based mmap for both types of mmap calls.

tasks/mm0/include/syscalls.h

@@ -24,7 +24,7 @@ struct sys_mmap_args {
 };
 
 void *sys_mmap(struct tcb *sender, void *start, size_t length, int prot,
-	       int flags, int fd, off_t offset);
+	       int flags, int fd, off_t pfn);
 
 int sys_munmap(struct tcb *sender, void *vaddr, unsigned long size);
 int sys_msync(struct tcb *task, void *start, unsigned long length, int flags);