Improve cancellations, randomness, and time

- Exhaustively document cancellation points
- Rename SIGCANCEL to SIGTHR just like BSDs
- Further improve POSIX thread cancellations
- Ensure asynchronous cancellations work correctly
- Elevate the quality of getrandom() and getentropy()
- Make futexes cancel correctly on OpenBSD 6.x and 7.x
- Add reboot.com and shutdown.com to examples directory
- Remove underscore prefix from awesome timespec_*() APIs
- Create assertions that help verify our cancellation points
- Remove bad timespec APIs (cmp generalizes eq/ne/gt/gte/lt/lte)

libc/sysv/consts/SIGEMT.s

@@ -1,2 +1,2 @@
 .include "o/libc/sysv/consts/syscon.internal.inc"
-.syscon sig,SIGEMT,0,7,7,7,7,0
+.syscon sig,SIGEMT,64,7,7,7,7,64

libc/sysv/consts/SIGINFO.s

@@ -1,2 +1,2 @@
 .include "o/libc/sysv/consts/syscon.internal.inc"
-.syscon sig,SIGINFO,0,29,29,29,29,0
+.syscon sig,SIGINFO,63,29,29,29,29,63

libc/sysv/consts/SIGTHR.s

@@ -1,2 +1,2 @@
 .include "o/libc/sysv/consts/syscon.internal.inc"
-.syscon sig,SIGCANCEL,32,7,65,7,33,32
+.syscon sig,SIGTHR,32,7,32,32,33,32

libc/sysv/consts/sig.h

@@ -7,7 +7,7 @@ COSMOPOLITAN_C_START_
 extern const int SIGABRT;
 extern const int SIGALRM;
 extern const int SIGBUS;
-extern const int SIGCANCEL;
+extern const int SIGTHR;
 extern const int SIGCHLD;
 extern const int SIGCONT;
 extern const int SIGEMT;
@@ -73,7 +73,7 @@ COSMOPOLITAN_C_END_
 #define SIGXFSZ   LITERALLY(25)
 
 #define SIGBUS    SYMBOLIC(SIGBUS)
-#define SIGCANCEL SYMBOLIC(SIGCANCEL)
+#define SIGTHR    SYMBOLIC(SIGTHR)
 #define SIGCHLD   SYMBOLIC(SIGCHLD)
 #define SIGCONT   SYMBOLIC(SIGCONT)
 #define SIGEMT    SYMBOLIC(SIGEMT)