LoadOrGenerateCertificate() takes two file path arguments, but then
uses the locations package to determine the actual path. Fix that
with a minimally invasive change, by using the arguments instead.
Factor out GenerateCertificate().
The only caller of this function is cmd/syncthing, which passes the
same values, so this is technically a no-op.
* lib/tlsutil: Make storing generated certificate optional. Avoid
temporary cert and key files in tests, keep cert in memory.
ec8a748514