Simplify cosmocc builds (#863)

- tcgetpgrp(STDIN_FILENO) should be equal to getpgrp() on Windows also,
  found while reading wget source code which uses this check to decide
  whether to print to stderr or to a file
- IN6_ADDR_ARE_EQUAL is a comparison macro used when IPV6 is allowed,
  found while reading CPython3.11 source code
- the changes in signal.h and addition of ucontext.h are because
  CPython3.11 source code expect sigaltstack to be available
- the sqlite3.mk change is because CPython3.11 requires sqlite3 to be
  built with -DOMIT_SHARED_CACHE
- unistd.h has getopt.h now, because some libraries like it there

libc/calls/tcgetpgrp.c

@@ -43,7 +43,7 @@ int tcgetpgrp(int fd) {
   } else if (IsBsd()) {
     rc = sys_ioctl(fd, TIOCGPGRP_bsd, &pgrp);
   } else if (IsWindows()) {
-    rc = getpid();
+    pgrp = rc = getpid();
   } else {
     rc = enosys();
   }