MINOR: log: Logging HTTP path only with %HPO

This patch adds a new logging variable '%HPO' for logging HTTP path only
(without query string) from relative or absolute URI.

For example:
log-format "hpo=%HPO hp=%HP hu=%HU hq=%HQ"

GET /r/1 HTTP/1.1
=>
hpo=/r/1 hp=/r/1 hu=/r/1 hq=

GET /r/2?q=2 HTTP/1.1
=>
hpo=/r/2 hp=/r/2 hu=/r/2?q=2 hq=?q=2

GET http://host/r/3 HTTP/1.1
=>
hpo=/r/3 hp=http://host/r/3 hu=http://host/r/3 hq=

GET http://host/r/4?q=4 HTTP/1.1
=>
hpo=/r/4 hp=http://host/r/4 hu=http://host/r/4?q=4 hq=?q=4

include/haproxy/log-t.h

@@ -169,6 +169,7 @@ enum {
 	LOG_FMT_HTTP_METHOD,
 	LOG_FMT_HTTP_URI,
 	LOG_FMT_HTTP_PATH,
+	LOG_FMT_HTTP_PATH_ONLY,
 	LOG_FMT_HTTP_QUERY,
 	LOG_FMT_HTTP_VERSION,
 	LOG_FMT_HOSTNAME,