Win32 changes

- [x] Cleaned up pcap-stdinc.h; include "IP6_Misc.h" for all targets.
- [x] Do be able to use libpcap as a static library, I've removed the `DllMain()` function.
    Hence ensure `wsockinit()` gets called for `WSAStartup()` and `InitializeCriticalSection()` only once.
    Not really needed, but added an atexit-handler for `WSACleanup()`.
- [x] Cleaned up the code around `pcap_compile()`; protect reentry with a critical-section.
- [x] Lots of trailing white-space changes.

On my TODO-list:
-  Add plugin support from Dustin Johnson (Dustin@Dustinj.us)
-  Add Remote capture support.
-  Be able to set data-link type for AirPcap devices; `p->set_datalink_op`