New coroutine 'crMaybeWait' macros, which may not return.

The crWaitUntil macros have do-while type semantics, i.e. they always
crReturn _at least_ once, and then perhaps more times if their
termination condition is still not met. But sometimes a coroutine will
want to wait for a condition that may _already_ be true - the key
examples being non-emptiness of a bufchain or a PacketQueue, which may
already be non-empty in spite of you having just removed something
from its head.

In that situation, it's obviously more convenient not to bother with a
crReturn in the first place than to do one anyway and have to fiddle
about with toplevel callbacks to make sure we resume later. So here's
a new pair of macros crMaybeWaitUntil{,V}, which have the semantics of
while rather than do-while, i.e. they test the condition _first_ and
don't return at all if it's already met.

ssh.c

@@ -362,6 +362,8 @@ enum {
 #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)
 
 struct Packet;