mail.sh: allow size_limit to be configured
Previously mail.sh has had a hard-coded limit of 256K for each
e-mail message it sends out.
In the era of megapixel camera images being sent as attachments,
the limit of 256K may be overly conservative.
Make the limit more easily configurable with a new option in
Config.pm $mailsh_sizelimit that sets the limit as the maximum
size in K byte units.
At the same time double the default limit from 256 K to 512 K
to avoid unnecessarily constraining the output while allowing
individual installations to easily customize the value.
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>