daemon: allow more than one host address given via --listen

Hi,
here is a new version of the two patches. The sockets are now in a
socketlist structure that is compatible with ALLOC_GROW().
Also if no socket could be created for one host/ip, git daemon prints
now an error but continues (in both ipv6 and noipv6 builds). Only if
no socket could be created at all, it will die.

Also I removed the string_list_clear (it leave back memory leaks form
the xstrdup'ed-strings). It would be happen just before the exit of the
application and just before the operating system cleans the memory. So
I hope, that it is okay to remove it. If not i will change it, to
really free all lists before exit.

Thanks
Alex