Eliminate cyclic locks in runtime

This change introduces a new deadlock detector for Cosmo's POSIX threads
implementation. Error check mutexes will now track a DAG of nested locks
and report EDEADLK when a deadlock is theoretically possible. These will
occur rarely, but it's important for production hardening your code. You
don't even need to change your mutexes to use the POSIX error check mode
because `cosmocc -mdbg` will enable error checking on mutexes by default
globally. When cycles are found, an error message showing your demangled
symbols describing the strongly connected component are printed and then
the SIGTRAP is raised, which means you'll also get a backtrace if you're
using ShowCrashReports() too. This new error checker is so low-level and
so pure that it's able to verify the relationships of every libc runtime
lock, including those locks upon which the mutex implementation depends.

libc/intrin/sigblock.c

@@ -16,14 +16,20 @@
 │ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
-#include "libc/sysv/consts/sig.h"
 #include "libc/calls/sig.internal.h"
 #include "libc/calls/struct/sigset.internal.h"
 #include "libc/dce.h"
 #include "libc/intrin/atomic.h"
 #include "libc/intrin/weaken.h"
+#include "libc/sysv/consts/sig.h"
 #include "libc/thread/tls.h"
 
+// since there's so many c library interfaces and system call wrappers
+// that always need to block signals we avoid the distraction of their
+// ftrace and strace output being muddied with sigprocmask lines. it's
+// usually better that sigprocmask only strace the user is calling it.
+// plus, since we have a very specific use case, this code goes faster
+
 struct Signals __sig;
 
 sigset_t __sig_block(void) {