I/O builtins and jq_handle_create()

This adds:

 - jq_handle_{create, create_stdio, get delete}() functions

   Note that handles have a type attribute.

 - read, write, fopen, and popen builtins

 - new options for jq(1):

    + --args (remaining arguments are strings, not files, passed as an array named $args);

    + --allow-{open, write, exec} to control whether the jq program can open files for read or write, or whether it can execute external programs.

Please review, comment.  I can push when it's ready, or you can.