Make improvements

- Document redbean's argon2 module
- Fix regressions in cthreads library
- Make testlib work better with threads
- Give the cthreads library lots of love
- Remove some of the stdio assembly code
- Implement getloadavg() across platforms
- Code size optimizations for errnos, etc.
- Only check for signals in main thread on Windows
- Make errnos for dup2 / dup3 consistent with posix

This change also fixes a bug in the argon2 module, where the NUL
terminator was being included in the hash encoded ascii string. This
shouldn't require any database migrations to folks who found this module
and productionized it, since the argon2 library treats it as a c string.

libc/calls/dup2.c

@@ -17,27 +17,47 @@
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/calls/calls.h"
+#include "libc/calls/internal.h"
 #include "libc/calls/strace.internal.h"
 #include "libc/calls/syscall-nt.internal.h"
 #include "libc/calls/syscall-sysv.internal.h"
 #include "libc/dce.h"
+#include "libc/sysv/errfuns.h"
 
 /**
  * Duplicates file descriptor, granting it specific number.
  *
+ * The `O_CLOEXEC` flag shall be cleared from the resulting file
+ * descriptor; see dup3() to preserve it.
+ *
+ * Unlike dup3(), the dup2() function permits oldfd and newfd to be the
+ * same, in which case the only thing this function does is test if
+ * oldfd is open.
+ *
  * @param oldfd isn't closed afterwards
  * @param newfd if already assigned, is silently closed beforehand;
  *     unless it's equal to oldfd, in which case dup2() is a no-op
  * @return new file descriptor, or -1 w/ errno
+ * @raise EBADF is oldfd isn't open
+ * @raise EBADF is newfd negative or too big
+ * @raise EINTR if a signal handler was called
  * @asyncsignalsafe
  * @vforksafe
  */
 int dup2(int oldfd, int newfd) {
   int rc;
-  if (oldfd == newfd) {
-    rc = newfd;
+  if (__isfdkind(oldfd, kFdZip)) {
+    rc = eopnotsupp();
   } else if (!IsWindows()) {
-    rc = sys_dup3(oldfd, newfd, 0);
+    rc = sys_dup2(oldfd, newfd);
+  } else if (newfd < 0) {
+    rc = ebadf();
+  } else if (oldfd == newfd) {
+    if (__isfdopen(oldfd)) {
+      rc = newfd;
+    } else {
+      rc = ebadf();
+    }
   } else {
     rc = sys_dup_nt(oldfd, newfd, 0, -1);
   }