Read the SMTP password from a file as opposed to the command-line argument

The [-xp <PASSWDORD>] option can be considered a security hole since allows anybody logged on the node to inspect the process arguments and intercept the value with "ps" os throug the /proc/ FS.

This patch introduces the additional option [-xp-file <FILE>] which tells the program to read the password from the <FILE>. This is more secure, since the file permissions can be set to restrict access according to Unix owner/groups.