Bug fix: Current serial number generation code does not produce a stable serial numbe...

Bug fix: Current serial number generation code does not produce a stable serial number for self-signed certificates

Squid always send the signing certificate to the ssl_crtd daemon even for
self-signed certificates because the signing certificate may used for cert
adaptation algorithms. The ssl_crtd currently ignore the signing certificate
in the case of self-signed certificates. This is has as result to
use a random number as serial number of generated certificate.

This patch also use 0 as serial number of the temporary intermediate certificate
used to generate the final serial number of the certificate, in the case of
signing certificate is not given.