Move simple_httpd out of picobsd, add HTTPD option (default OFF)

picobsd/tinyware has had this compact HTTPD server for a long time, and some
people do use it. Move it out into usr.sbin well in advance of any action
being taken on picobsd.

This has been gated behind an HTTPD option defaulted to *off*, primarily for
two reasons:
1.) This code likely needs a good audit, as it's been living off in picobsd

land for a long time, and

2.) We don't currently ship an httpd and this may not be a welcome surprise.

Reviewed by: eugen
Differential Revision: https://reviews.freebsd.org/D21724