Refactor to rename "Rhizome HTTP server" to "HTTPD server"

Move HTTPD start/stop/dispatch logic from rhizome_httpd.c into new
httpd.c

Rename config.debug.httpd -> config.debug.http_server
Rename config.debug.rhizome_httpd -> config.debug.httpd
Rename config.debug.rhizome_nohttptx -> config.debug.nohttptx
Rename "rhizome_http.h" to "httpd.h"