security/softether: cleanup Makefile

I'm working on update security/softether-devel. However, this port and
-devel port were created and maintained by other people. So there's no
consistency between these ports. I'm going to renew -devel port this
port based. Before updating -devel, cleanup existing Makefile issues
on this port:

• add LICENSE_FILE
• add ONLY_FOR_ARCHS_REASON
• avoid useless use of +=
• do patch in post-patch, not post-extract
• do everything in do-install
• remove MAKE_JOBS_UNSAFE as it actually build now
• simplify DOT2UNIX_FILES
• use sysrc rather than editing rc.conf in pkg-message
• use tab after =

Approved by: mentors (implicit)