Start working on memory-mapped files.

First of all, change sys$mmap to take a struct SC_mmap_params since our
sycsall calling convention can't handle more than 3 arguments.

This exposed a bug in Syscall::invoke() needing to use clobber lists.
It was a bit confusing to debug. :^)

LibC/pwd.cpp

@@ -32,7 +32,7 @@ void setpwent()
             perror("open /etc/passwd");
         }
         assert(__pwdb_stream);
-        __pwdb_entry = (struct passwd_with_strings*)mmap(nullptr, getpagesize());
+        __pwdb_entry = (struct passwd_with_strings*)mmap(nullptr, getpagesize(), PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_PRIVATE, 0, 0);
         set_mmap_name(__pwdb_entry, getpagesize(), "setpwent");
     }
 }