Add pam_silent sudoers option.

Inspired by PR #368
GitHub issue #216

plugins/sudoers/defaults.c

@@ -661,6 +661,7 @@ init_defaults(void)
     def_set_utmp = true;
     def_pam_acct_mgmt = true;
     def_pam_setcred = true;
+    def_pam_silent = true;
     def_syslog_maxlen = MAXSYSLOGLEN;
     def_case_insensitive_user = true;
     def_case_insensitive_group = true;