Fix fork() crash on Windows

On Windows, sometimes fork() could crash with message likes:

    fork() ViewOrDie(170000) failed with win32 error 487

This is due to a bug in our file descriptor inheritance. We have cursors
which are shared between processes. They let us track the file positions
of read() and write() operations. At startup they were being mmap()ed to
memory addresses that were assigned by WIN32. That's bad because Windows
likes to give us memory addresses beneath the program image in the first
4mb range that are likely to conflict with other assignments. That ended
up causing problems because fork() needs to be able to assume that a map
will be possible to resurrect at the same address. But for one reason or
another, Windows libraries we don't control could sneak allocations into
the memory space that overlap with these mappings. This change solves it
by choosing a random memory address instead when mapping cursor objects.

libc/runtime/runtime.h

@@ -95,6 +95,7 @@ int ftrace_install(void) libcesque;
 int ftrace_enabled(int) libcesque;
 int strace_enabled(int) libcesque;
 void __print_maps(size_t) libcesque;
+void __print_maps_win32(void) libcesque;
 void __printargs(const char *) libcesque;
 /* builtin sh-like system/popen dsl */
 int _cocmd(int, char **, char **) libcesque;