tcpdump-stdinc.h cleanup

"bittypes.h" from WinPcap's Win32/include directory is getting old and messes up the definitions in <stdint.h> already included in tcpdump-stdint.h. E.g. a problem is that when tcpdump-stdinc.h does:
```
  #ifndef int8_t
  #define int8_t  signed char
  #endif
```
the "bittypes.h" included via pcap's headers, the expansion becomes:
```
  #ifndef HAVE_U_INT8_T
  typedef unsigned char u_int8_t;
  typedef signed char signed char;
  #endif /* HAVE_U_INT8_T */
```
That's why "bittypes.h" MUST come early in tcpdump-stdinc.h.

Simplified the `#ifdef` around old-names for MSVC and `UNALIGNED`.