Embed cocmd.com interpreter for system() / open()

This change lets you use system() in an easier and portable way. The
problem with the call in the past has always been that bourne and
cmd.com on Windows have less than nothing in common, so pretty much the
only command system() could be used for across platforms was maybe echo.
cmd.exe is also a security liability due to its escaping rules.

Since cocmd.com implements 85% of what we need from bourne, in a really
tiny way, it makes perfect sense to be embedded in these functionss. We
get a huge performance boost too.

Fixes #644

libc/stdio/popen.c

@@ -29,6 +29,16 @@
 
 /**
  * Spawns subprocess and returns pipe stream.
+ *
+ * This embeds the cocmd.com shell interpreter which supports a limited
+ * subset of the bourne shell that's significantly faster:
+ *
+ * - pipelines
+ * - single quotes
+ * - double quotes
+ * - input redirection, e.g. `<path`
+ * - output redirection, e.g. `>path`, `>>append`, `2>err.txt, `2>&1`
+ *
  * @see pclose()
  */
 FILE *popen(const char *cmdline, const char *mode) {