Add option to disable the S/MIME efail security

* src/common_indep.h (opt): Add option.
* src/mailitem-events.cpp (REPLY/FORWARD/REPLYALL): Check
for new option.
* src/main.c (read_options): Read new config option.

Allow sysadmins to disable S/MIME Efail security as
an option. It is of course disabled by default, hidden
and should only be used in scenarios which are fully
under control or which use organizational mesasures
to defend against manipulation of unsigned S/MIME
messages.