Fix the formating of the e-mail subject

Fix the formating of the e-mail subject

The date in the subject line of the e-mail was produced with asctime.
But that function append a \n at the end of the date. The subject line
was therefore followed by a blank line as mailx was concerned and that
would mess with the header of the e-mail.

This patch render the subject line using strftime. It avoid the line
break and allow mailx to add additional headers from the command line.