Greatly expand system() shell code features

The cosmopolitan command interpreter now has 13 builtin commands,
variable support, support for ; / && / || syntax, asynchronous support,
and plenty of unit tests with bug fixes.

This change fixes a bug in posix_spawn() with null envp arg. strace
logging now uses atomic writes for scatter functions. Breaking change
renaming GetCpuCount() to _getcpucount(). TurfWar is now updated to use
the new token bucket algorithm. WIN32 affinity masks now inherit across
fork() and execve().

libc/str/strchr.c

@@ -61,6 +61,7 @@ noasan static inline const char *strchr_sse(const char *s, unsigned char c) {
  * @return pointer to first instance of c or NULL if not found
  *     noting that if c is NUL we return pointer to terminator
  * @asyncsignalsafe
+ * @vforksafe
  */
 char *strchr(const char *s, int c) {
   const char *r;