SMP support, part 1: Essential non-caching functionality.

Added workers squid.conf option to specify how many main Squid
processes to fork and maintain. Zero means old no-daemon mode.
One means the old non-SMP mode.

Added support for process_name and process_number macros and
if-statement conditionals in squid.conf. Search for .pre changes for
documented details. These features allow the admin to configure each
worker process differently if needed.

Support multiple workers listening on the same HTTP[S] port (port
sharing). This allows multiple workers to split the load without any
special rules.

Support or prohibit port sharing for WCCP, DNS, ICP, HTCP, SNMP, and
Ident protocols, depending on protocol-specific restrictions. Sharing is
implemented by registering listening socket descriptors with the
Coordinator process and obtaining them from the Coordinator as needed.
Here are protocol-specific notes:

   WCCP: Restricted to the Coordinator process due to how WCCP works.
   Workers do not need access to the WCCP code.

   DNS: Done by each worker with no sharing. Fixed source ports not
   supported unless each worker is given its own outgoing address
   because we do not want to match outgoing queries and incoming
   responses across processes.

   SNMP: Workers share incoming and outgoing sockets.

   ICP and HTCP _clients_: Cannot be supported in SMP environment
   unless each process has its own address (i.e., unique IP address
   and/or unique [ICP] port) because we do not want to match outgoing
   queries and incoming responses across processes.

   ICP and HTCP _servers_: share listening sockets.

   Ident clients do not need to share sockets because they use
   unique ports.

Support management signals (squid -k ...) in SMP mode, acting as a
single Squid instance.

Refork dying workers, similar to how we reforked dying process in
non-SMP daemon mode.

Merged from lp smp branch r10306.

diff --cc configure.in
Simple merge