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/grp.cpp

@@ -31,7 +31,7 @@ void setgrent()
             perror("open /etc/group");
         }
         assert(__grdb_stream);
-        __grdb_entry = (struct group_with_strings*)mmap(nullptr, getpagesize());
+        __grdb_entry = (struct group_with_strings*)mmap(nullptr, getpagesize(), PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_PRIVATE, 0, 0);
         set_mmap_name(__grdb_entry, getpagesize(), "setgrent");
     }
 }