Make improvements

- Improved async signal safety of read() particularly for longjmp()
- Started adding cancel cleanup handlers for locks / etc on Windows
- Make /dev/tty work better particularly for uses like `foo | less`
- Eagerly read console input into a linked list, so poll can signal
- Fix some libc definitional bugs, which configure scripts detected

libc/sock/sendfile.c

@@ -219,3 +219,5 @@ ssize_t sendfile(int outfd, int infd, int64_t *opt_in_out_inoffset,
          DescribeInOutInt64(rc, opt_in_out_inoffset), uptobytes, rc);
   return rc;
 }
+
+__weak_reference(sendfile, sendfile64);