context/initcontext.go: Change tlscerts semantics from "add" to "replace".
context/initcontext.go: Change tlscerts semantics from "add" to "replace".

I prefer not to trust as large a set of RootCAs as the typical operating
system.  It suffices for rc to set tlscerts: {$HOME}/upspin/RootCA where
that directory contains just
  DST_Root_CA_X3.pem
copied from /etc/ssl/certs or wherever, plus whatever other CA you want
for your own non-upspin.io servers.

The default behavior remains that upspin will trust all well known CAs.

Fix issue #68.

Change-Id: I3c3fdf7de79a45f2e54122e13f560632da1f4c8d
Reviewed-on: https://upspin-review.googlesource.com/6323
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@google.com>
Reviewed-by: David Presotto <516b9783fca517eecbd1d064da2d165310b19759@google.com>

upspin/upspin,upspin/upspin