Squid-2.6 comm_select_simple.c added

Based on the experiences from rewriting comm_select_win32.c and reading
SUSv3/IEEE1003.1 2004 edition I now "wrote" yet another select
implementation for 2.6: comm_select_simple. This should work on any
system implementing select() per the current POSIX specs, including
Windows and a number of other exotic systems I suppose.

Discussion: The normal comm_select.c assumes fd_set has a certain
structure (fds_bits[]) and optimizes the scan based on that. However,
this field is no longer defined by the standards, and even in the older
standards where it was defined it's definition is not crystal clear
(bitorder undefined).

Configure option: --enable-select-simple

Regards
Henrik