Fix some NetBSD-specific build problems.

Use proper identification macros for NetBSD and its defined
architectures. While at it, define the correct constant in the Amiga
block.  With these changes sniffit can build and start (but still does
not work correctly) on at least one architecture:

NetBSD octeon 10.0 NetBSD 10.0 (OCTEON) #0: Thu Mar 28 08:33:33 UTC 2024
  [email protected]:/usr/src/sys/arch/evbmips/compile/OCTEON
  evbmips

The configure script does not use pkg-config and thus needs to be
invoked as follows to detect pkgsrc ncurses:

CFLAGS=$(pkg-config --cflags ncurses) \
  LDFLAGS=$(pkg-config --libs ncurses) ./configure

src/sn_data.h

@@ -60,19 +60,24 @@ char *NETDEV[]={"ln"};
 int HEADSIZE[]={14};
 #endif
 
-#ifdef NETBSD
-#ifdef i386
+#ifdef __NetBSD__
+#if defined(i386) || defined(__amd64__)
 #define NETDEV_NR      21
 char *NETDEV[]={"ppp","ai","de","ec","ef","eg","el","en","ep","fe","fea","fpa","fxp","ix","iy","lc","le","ne","sm","tl","we"};
 int HEADSIZE[]={4    ,14  ,14  ,14  ,14  ,14  ,14  ,14  ,14  ,14  ,14   ,14   ,14   ,14  ,14  ,14  ,14  ,14  ,14  ,14  ,14  };
-#elif defined(sparc)
+#elif defined(__sparc__)
 #define NETDEV_NR      3
 char *NETDEV[]={"ppp","le","ie"};
 int HEADSIZE[]={4    ,14  ,14};
-#elif defined(amiga)
-#define ETH_DEV_NR      6
+#elif defined(__m68k__)
+#define NETDEV_NR      6
 char *NETDEV[]={"ppp","bah","ed","es","le","qn"};
 int HEADSIZE[]={4    ,14  ,14   ,14  ,14  ,14};
+#elif defined(__mips64) && defined(__MIPSEB__)
+/* Assume Octeon. */
+#define NETDEV_NR      1
+char *NETDEV[]={"cnmac"};
+int HEADSIZE[]={14};
 #else
 #error Unknown network devices for this NetBSD architecture.
 #endif