Reorganise supported signature algorithm extension processing.

Only store encoded versions of peer and configured signature algorithms.
Determine shared signature algorithms and cache the result along with NID
equivalents of each algorithm.
(backport from HEAD)

ssl/ssl.h

@@ -2532,6 +2532,7 @@ void ERR_load_SSL_strings(void);
 #define SSL_R_NO_RENEGOTIATION				 339
 #define SSL_R_NO_REQUIRED_DIGEST			 324
 #define SSL_R_NO_SHARED_CIPHER				 193
+#define SSL_R_NO_SHARED_SIGATURE_ALGORITHMS		 376
 #define SSL_R_NO_SRTP_PROFILES				 359
 #define SSL_R_NO_VERIFY_CALLBACK			 194
 #define SSL_R_NULL_SSL_CTX				 195