Dodo Nethub support (#445)

* added support for XNS networking via Dodo-Nethub

* NetHub connection now optional (only if -nh-host is given); released NetHub-related changes to 'ether.c' to the public domain

* Added file using-dodo-networking-with-maiko.md

Documentation for building and using the Dodo-networking addition to Maiko

* Added support for running Maiko unter cygwin/x86_64-x

* Migrate Addr68k/NativeAlignment{2,4} in Nethub code, move timer/async defines to platform.h

* added missing include <netinet/in.h> for FreeBSD

* updated 'compile-flags' with added flags

* splitted ether.c in 3 (_common, _sunos, _nethub)

* reworks/modifications for nbriggs' pull-request review comments

* addintional additions for nbriggs' pull-request review comments

* get the Lisp packet lengths with 'LispInt2CInt'

* renamed variables in dblwordsSwap to indicate it's about double-words

* fixed wrong preprocessor directive unnoticed by clang

* added networking choice option to cmake build, fix to printf warning

- for cmake specify the networking to use with -DMAIKO_NETWORK_TYPE=<type>
- with <type> one of: NONE, SUN_DLPI, SUN_NIT, NETHUB
- e.g.: cmake .. -DMAIKO_NETWORK_TYPE=NETHUB

* integrated improvement to sendPacket() proposed by nbriggs

* integrated fix for SIGBUS on 32-bit big-endian, provided by nbriggs

* MAIKO_ENABLE_ETHERNET should not be unconditionally set on Solaris systems

* Receiving an ethernet packet is an ether interrupt but not an i/o interrupt.

Co-authored-by: dev hawala <devhawala@x.y>
Co-authored-by: Nick Briggs <nicholas.h.briggs@gmail.com>

src/xc.c

@@ -162,6 +162,17 @@ static const int n_mask_array[16] = {
 
 extern int TIMER_INTERVAL;
 
+#if defined(MAIKO_EMULATE_TIMER_INTERRUPTS) || defined(MAIKO_EMULATE_ASYNC_INTERRUPTS)
+
+#  if !defined(MAIKO_TIMER_ASYNC_EMULATION_INSNS_COUNTDOWN)
+#    define MAIKO_TIMER_ASYNC_EMULATION_INSNS_COUNTDOWN 20000
+#  endif
+
+int insnsCountdownForTimerAsyncEmulation = MAIKO_TIMER_ASYNC_EMULATION_INSNS_COUNTDOWN;
+static int pseudoTimerAsyncCountdown = MAIKO_TIMER_ASYNC_EMULATION_INSNS_COUNTDOWN;
+
+#endif
+
 void dispatch(void) {
   InstPtr pccache;
 
@@ -260,6 +271,17 @@ nextopcode:
 #endif /* PCTRACE */
 
   /* quick_stack_check();*/ /* JDS 2/12/98 */
+  
+#if defined(MAIKO_EMULATE_TIMER_INTERRUPTS) || defined(MAIKO_EMULATE_ASYNC_INTERRUPTS)
+  if (--pseudoTimerAsyncCountdown <= 0) {
+	  Irq_Stk_Check = 0;
+	  Irq_Stk_End = 0;
+#if defined(MAIKO_EMULATE_ASYNC_INTERRUPTS)
+	  IO_Signalled = TRUE;
+#endif
+	  pseudoTimerAsyncCountdown = insnsCountdownForTimerAsyncEmulation;
+  }
+#endif
 
   switch (Get_BYTE_PCMAC0) {
     case 000: