feat: add missing HTTP status codes to httpresp()

[mgrossmann]

Summary

httpresp() in httpresp.c maps HTTP status codes to reason phrases. Unknown codes fall through to the default case which sends 500 Internal Server Error. This causes CGI modules that send valid but uncommon status codes to have their responses silently replaced with 500.

Missing codes

The following standard HTTP status codes are not supported:

Code	Reason Phrase	Use case
405	Method Not Allowed	Router sends this for unsupported methods
409	Conflict	Resource already exists, directory not empty
414	URI Too Long	Path name exceeds limits
507	Insufficient Storage	Filesystem full, no inodes

Impact

mvsMF's USS file API needs 409 (Conflict) for duplicate file/directory detection and 507 for filesystem-full conditions. Currently these are remapped to 400/500 as a workaround, losing semantic precision.

Suggested fix

Add the missing codes to the switch statement in httpresp():

case 405: reason = "Method Not Allowed"; break;
case 409: reason = "Conflict"; break;
case 414: reason = "URI Too Long"; break;
case 507: reason = "Insufficient Storage"; break;