check_smtp: Strip angle brackets from -f argument, if provided

Apparently some people used -f '<nagios@example.com>' to work around
the bug I just fixed in the MAIL FROM: command generation.  Although
the resulting command wasn't RFC-compliant, it was working with some
MTAs, so let's continue to support this syntax now that we generate
RFC-compliant commands.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

plugins/check_smtp.c

@@ -521,7 +521,8 @@ process_arguments (int argc, char **argv)
 			localhostname = strdup(optarg);
 			break;
 		case 'f':									/* from argument */
-			from_arg = optarg;
+			from_arg = optarg + strspn(optarg, "<");
+			from_arg = strndup(from_arg, strcspn(from_arg, ">"));
 			smtp_use_dummycmd = 1;
 			break;
 		case 'A':