Make fixes and improvements

- Document more compiler flags
- Expose new __print_maps() api
- Better overflow checking in mmap()
- Improve the shell example somewhat
- Fix minor runtime bugs regarding stacks
- Make kill() on fork()+execve()'d children work
- Support CLONE_CHILD_CLEARTID for proper joining
- Fix recent possible deadlock regression with --ftrace

libc/nexgen32e/nt2sysv.S

@@ -32,6 +32,8 @@
 __nt2sysv:
 	push	%rbp
 	mov	%rsp,%rbp
+//	TODO(jart): We should probably find some way to use our own
+//	            stack when Windows delivers signals ;_;
 	.profilable
 	sub	$0x100,%rsp
 	push	%rbx