pveproxy: allow to request debug sources directly over GET parameter

pveproxy: allow to request debug sources directly over GET parameter

Avoids the need to stop the current pveproxy and restart it in debug
mode, if one wants to get the debug un-minified sources for ExtJS,
for example.

For a local PVE instance the following URL would ensure you load the
debug sources:
https://localhost:8006/?debug#v1:0:18:4:25:19::8:20:28

For convenience allow to omit the parameters value, and default to
true, iow, ?debug and ?debug=1 are the same, one could force it off -
even if the server was started in debug mode - by using ?debug=0

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>