Display a different prompt for sysops (that includes the [D]elete option)

This addresses the gitlab issue #130.

Also, prompt for confirmation when [D]eleting the file.

Also, allow modopts.ini customization of this script via keys
in the new (optional) [automsg] section:

prompt (defaults to text.dat AutoMsg)
sysop_prompt (defaults to hard-coded string)
intro (defaults to hard-coded string)
header_fmt (defaults to text.dat AutoMsgBy)
user_fmt (defaults to hard-coded string)
line_fmt (defaults to hard-coded string)
max_line_len (defaults to 76)