Move some ssh.c declarations into header files.

ssh.c has been an unmanageably huge monolith of a source file for too
long, and it's finally time I started breaking it up into smaller
pieces. The first step is to move some declarations - basic types like
packets and packet queues, standard constants, enums, and the
coroutine system - into headers where other files can see them.

sshcr.h

@@ -0,0 +1,54 @@
+/*
+ * Coroutine mechanics used in PuTTY's SSH code.
+ */
+
+#ifndef PUTTY_SSHCR_H
+#define PUTTY_SSHCR_H
+
+/*
+ * If these macros look impenetrable to you, you might find it helpful
+ * to read
+ * 
+ *   https://www.chiark.greenend.org.uk/~sgtatham/coroutines.html
+ * 
+ * which explains the theory behind these macros.
+ * 
+ * In particular, if you are getting `case expression not constant'
+ * errors when building with MS Visual Studio, this is because MS's
+ * Edit and Continue debugging feature causes their compiler to
+ * violate ANSI C. To disable Edit and Continue debugging:
+ * 
+ *  - right-click ssh.c in the FileView
+ *  - click Settings
+ *  - select the C/C++ tab and the General category
+ *  - under `Debug info:', select anything _other_ than `Program
+ *    Database for Edit and Continue'.
+ */
+
+#define crBegin(v)      { int *crLine = &v; switch(v) { case 0:;
+#define crBeginState    crBegin(s->crLine)
+#define crStateP(t, v)                          \
+    struct t *s;                                \
+    if (!(v)) { s = (v) = snew(struct t); s->crLine = 0; }      \
+    s = (v);
+#define crState(t)      crStateP(t, ssh->t)
+#define crFinish(z)     } *crLine = 0; return (z); }
+#define crFinishV       } *crLine = 0; return; }
+#define crFinishFree(z) } sfree(s); return (z); }
+#define crFinishFreeV   } sfree(s); return; }
+#define crReturn(z)     \
+        do {\
+            *crLine =__LINE__; return (z); case __LINE__:;\
+        } while (0)
+#define crReturnV       \
+        do {\
+            *crLine=__LINE__; return; case __LINE__:;\
+        } while (0)
+#define crStop(z)       do{ *crLine = 0; return (z); }while(0)
+#define crStopV         do{ *crLine = 0; return; }while(0)
+#define crWaitUntil(c)  do { crReturn(0); } while (!(c))
+#define crWaitUntilV(c) do { crReturnV; } while (!(c))
+#define crMaybeWaitUntil(c) do { while (!(c)) crReturn(0); } while (0)
+#define crMaybeWaitUntilV(c) do { while (!(c)) crReturnV; } while (0)
+
+#endif /* PUTTY_SSHCR_H */