MASTER_SITE dl.farsightsecurity.com only allows https downloads.
Mirror the distfile on LOCAL as a http backup for clients that
can't use https. (Suggested by mat@)

Add MANPAGES option (default YES) because generating the man page
adds a significant number of build dependencies.

Add a comment explaining that the DOXYGEN=NO option gets ignored
if doxygen is installed.

Use option helpers (suggested by mat@).

Use PORTDOCS=* to avoid the need to change pkg-plist if doxygen
changes.

Use INSTALL_TARGET=install-strip instead of ${STRIP_CMD}
(suggested by marino@).

Simplify examples installation.

Differential Revision: https://reviews.freebsd.org/D1484
Approved by: mat (mentor)
Sponsored by: Farsight Security, Inc.