add support for RFC976 From_ lines

RFC976 describes these as:

	... the "source path" (normally
	represented in one or more lines at the beginning of the message
	beginning either "From " or ">From ", sometimes called "From_
	lines".)

This fixes an issue where emails generated by cron (on some systems) would have their headers in the mail body.

If the first line starts with "From " or ">From " the line is skipped instead of parsing everything as body.

mail.c

@@ -354,6 +354,7 @@ readmail(struct queue *queue, int nodot, int recp_from_header)
 	int had_from = 0;
 	int had_messagid = 0;
 	int had_date = 0;
+	int had_first_line = 0;
 	int had_last_line = 0;
 	int nocopy = 0;
 
@@ -391,6 +392,15 @@ readmail(struct queue *queue, int nodot, int recp_from_header)
 			line[linelen + 1] = 0;
 			had_last_line = 1;
 		}
+		if (!had_first_line) {
+			/*
+			 * Ignore a leading RFC-976 From_ or >From_ line mistakenly
+			 * inserted by some programs.
+			 */
+			if (strprefixcmp(line, "From ") == 0 || strprefixcmp(line, ">From ") == 0)
+				continue;
+			had_first_line = 1;
+		}
 		if (!had_headers) {
 			/*
 			 * Unless this is a continuation, switch of