[BUG] cfgparser/stats: fix error message

[BUG] cfgparser/stats: fix error message

Fix the error message by unification and goto, previously we had
two independent lists of supported keywords and were raporting 'stats'
instead of a wrong keyword.

Code:
 stats wrong-keyword
 stats

Before:
 [ALERT] 005/163032 (27175) : parsing [haproxy.cfg:248] : unknown stats parameter 'stats' (expects 'hide-version', 'uri', 'realm', 'auth' or 'enable').
 [ALERT] 005/163032 (27175) : parsing [haproxy.cfg:249] : 'stats' expects 'uri', 'realm', 'auth', 'scope' or 'enable', 'hide-version', 'show-node', 'show-desc', 'show-legends'.

After:
 [ALERT] 005/162841 (22710) : parsing [haproxy.cfg:248]: unknown stats parameter 'wrong-keyword', expects 'uri', 'realm', 'auth', 'scope', 'enable', 'hide-version', 'show-node', 'show-desc' or 'show-legends'.
 [ALERT] 005/162841 (22710) : parsing [haproxy.cfg:249]: missing keyword in 'stats', expects 'uri', 'realm', 'auth', 'scope', 'enable', 'hide-version', 'show-node', 'show-desc' or 'show-legends'.