Do not interpret TFTP filename as absolute url.

[ebikt]

With -http-append-path TFTP request //foo/bar discars path portion of base url and http://foo/bar made request to server foo completely ignoring base url. This patch validates base url on start and does not interpret TFTP filename in any special way. It just strips all leading slashes and appends it to base url when -http-append-path is enabled.

Reproducing error on previous version of tftp-http-proxy: curl tftp://localhost/http://example.com/ (provided that tftp-http-proxy has access to internet).

[bwalex]

Looks alright - thanks! I'll give it a spin on my side, hopefully today or tomorrow, and will merge it after that.