Avoiding the issue with https and 'fetch.type svn'

[Bryan Blackburn]

Ports which use 'fetch.type svn' and have an https URL (eg, tickets #17419,
#18429, and possibly #18583) will quite likely fail for people in the fetch
stage.  svn from the current subversion port (version 1.6.1) has the
--trust-server-cert option to avoid this, but /usr/bin/svn on 10.5 is too
old to have it.  Unless we are going to add subversion as a dependency for
all ports setting svn as the fetch type, this is a problem.

The maintainer of the flashdot port (Tobias Elze) mentioned one way this can
be worked around in the Portfile, using "echo p | svn ls ${svn.url}" to get
the cert accepted first.  So I added this in pre-fetch as a test and it
works quite nicely.

This can obviously be added to any port needing it, or perhaps even to base,
but is a bit kludgy overall; does anyone have any better ideas to fix this
or shall we start updating some Portfiles?

Bryan