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/stdio/unlocked/stdio_unlock.S

@@ -38,6 +38,7 @@
 stdio_unlock:
 	push	%rbp
 	mov	%rsp,%rbp
+	.profilable
 
 //	acquires mutex
 	push	%rcx