Improve synchronization

- Fix bugs in kDos2Errno definition
- malloc() should now be thread safe
- Fix bug in rollup.com header generator
- Fix open(O_APPEND) on the New Technology
- Fix select() on the New Technology and test it
- Work towards refactoring i/o for thread safety
- Socket reads and writes on NT now poll for signals
- Work towards i/o completion ports on the New Technology
- Make read() and write() intermittently check for signals
- Blinkenlights keyboard i/o so much better on NT w/ poll()
- You can now poll() files and sockets at the same time on NT
- Fix bug in appendr() that manifests with dlmalloc footers off

test/libc/stdio/vappendf_test.c

@@ -17,10 +17,15 @@
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/bits/bits.h"
+#include "libc/intrin/kprintf.h"
 #include "libc/stdio/append.internal.h"
 #include "libc/testlib/ezbench.h"
 #include "libc/testlib/testlib.h"
 
+static void PrintMemory(void *p) {
+  kprintf("%#.*hhs%n", malloc_usable_size(p), p);
+}
+
 TEST(vappendf, test) {
   char *b = 0;
   ASSERT_NE(-1, appendf(&b, "hello "));
@@ -137,6 +142,7 @@ TEST(appendr, testExtend_zeroFills) {
 TEST(appendr, testAbsent_allocatesNul) {
   char *b = 0;
   ASSERT_NE(-1, appendr(&b, 0));
+  ASSERT_BINEQ(u" ", b);
   EXPECT_EQ(0, appendz(b).i);
   ASSERT_BINEQ(u" ", b);
   free(b);