url encoding a string with base system tools

[Aryeh Friedman]

I am creating a new port and part of the install procedure is the
install script needs to send some data to a web server in the form of
http://.....?XXXXX where XXX is the url encoded plain text (can
include any ascii printable character) that needs to be sent.

Since this for a port I want to do the plain to url encoded text with
tools *ONLY* found in the base freebsd install (no ports).   How would
I do this?