net: use model NSS socket to save setting the cipher order every time.

Because setting the cipher order is a quadratic operation, this change does it
once at startup and exposes a model NSS socket that is then cloned when we need
new ones.

When writing, I also found a bug in SSL_CipherOrderSet which I hadn't noticed.
Because SSL_ImplementedCiphers includes, at the end, several SSLv2 cipher
suites (SSL_EN_*) that aren't included in ssl3con.c:cipherSuites, the function
was always failing because it couldn't find those cipher suites. However, it
was failing after having made the needed updates, so everything functioned
correctly.

This change alters SSL_CipherOrderSet to ignore unknown cipher suites because
it's not clear how else users of NSS are supposed to use it given the
SSL_ImplementedCiphers includes these unknown cipher suites.

BUG=310768
R=wtc@chromium.org

Review URL: https://codereview.chromium.org/100393002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239340 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239340