slirp-related compilation changes

A set of changes to allow slirp functionality to be compiled into FS-UAE.

I've added a couple of defines WITH_UAENET_PCAP and HAVE_STRUCT_IN_ADDR_S_UN, which I have already also added to od-win32/sysconfig.h.

I also moved MAX_TOTAL_NET_DEVICES from win32_uaenet.h to another header file (to make it available to FS-UAE), and similarly moved uaenet_gotdata/uaenet_getdata prototypes to sana2.cpp.