Automatically download if file is an rsync URL

Automatically download if file is an rsync URL

rsync cannot download into standard output... which means rsync'd files
cannot be elegantly piped as standard output to --mode=print. So either
the rsync has to be done manually by the user... or --mode=print has to
do it internally by itself.

And looking at the code that resulted... I now wish I had gone with the
former option. Because of the long overdue cache refactors, the user
needs to include --tal for this rsync to be compatible with the cache.
This sucks.

As a workaround, Fort will rsync into /tmp if --tal and/or --local-cache
aren't supplied:

	$ fort --mode=print \
		--validation-log.enabled \
		--validation-log.level debug \
		rsync://a.b.c/d/CRL.crl
	...
	May 10 13:32:44 DBG [Validation]: Executing rsync:
	May 10 13:32:44 DBG [Validation]:     rsync
	May 10 13:32:44 DBG [Validation]:     ...
	May 10 13:32:44 DBG [Validation]:     rsync://a.b.c/d/CRL.crl
	May 10 13:32:44 DBG [Validation]:     /tmp/fort-Q7tMhz/CRL.crl
	...
	{
	"tbsCertList": {
		"version": 1,
	...