Add epoll and do more release readiness changes

This change also pays off some of the remaining technical debt with
stdio, file descriptors, and memory managemnt polyfills.

libc/stdio/feof.c

@@ -19,6 +19,9 @@
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/stdio/stdio.h"
 
+/**
+ * Returns true if stream is in end-of-file state.
+ */
 int feof(FILE *f) {
   return f->state == -1;
 }