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

tool/build/runit.c

@@ -314,8 +314,8 @@ static void Send(const void *output, size_t outputsize) {
   static z_stream zs;
   static char zbuf[4096];
   if (!once) {
-    CHECK_EQ(Z_OK, deflateInit2(&zs, Z_DEFAULT_COMPRESSION, Z_DEFLATED,
-                                MAX_WBITS, DEF_MEM_LEVEL, Z_DEFAULT_STRATEGY));
+    CHECK_EQ(Z_OK, deflateInit2(&zs, 4, Z_DEFLATED, MAX_WBITS, DEF_MEM_LEVEL,
+                                Z_DEFAULT_STRATEGY));
     once = true;
   }
   zs.next_in = output;