Gracefully fail FAPI init when it's not compiled in

Instead of emitting:

   WARNING: Getting tokens from fapi backend failed.

errors when FAPI support is not compiled in gracefully fail the FAPI
init and don't log any warnings. We'll still produce a message
indicating this is what's happened in verbose mode, but normal operation
no longer gets an unnecessary message.

Fixes #792

Signed-off-by: Jonathan McDowell <noodles@earth.li>

src/lib/backend.c

@@ -53,7 +53,9 @@ CK_RV backend_init(void) {
             LOGE(msg);
             return rv;
         }
-        LOGW(msg);
+        if (rv != CKR_FUNCTION_NOT_SUPPORTED) {
+            LOGW(msg);
+        }
     } else {
         fapi_init = true;
     }

src/lib/backend_fapi.c

@@ -977,7 +977,8 @@ CK_RV backend_fapi_token_changeauth(token *tok, bool user, twist toldpin, twist
 
 CK_RV backend_fapi_init(void) {
 
-	return CKR_OK;
+	LOGV("FAPI not enabled, failing init");
+	return CKR_FUNCTION_NOT_SUPPORTED;
 }
 
 CK_RV backend_fapi_destroy(void) {