Update chain building function.

Don't clear verification errors from the error queue unless
SSL_BUILD_CHAIN_FLAG_CLEAR_ERROR is set.

If errors occur during verification and
SSL_BUILD_CHAIN_FLAG_IGNORE_ERROR is set return 2 so applications can
issue warnings.

(Imported from upstream's 2dd6976f6d02f98b30c376951ac38f780a86b3b5)