Move MAX_BACKENDS to procnumber.h

MAX_BACKENDS influences many things besides postmaster. I e.g. noticed that we
don't have static assertions ensuring BUF_REFCOUNT_MASK is big enough for
MAX_BACKENDS, adding them would require including postmaster.h in
buf_internals.h which doesn't seem right.

While at that, add MAX_BACKENDS_BITS, as that's useful in various places for
static assertions (to be added in subsequent commits).

Reviewed-by: Thomas Munro <thomas.munro@gmail.com>
Discussion: https://postgr.es/m/wptizm4qt6yikgm2pt52xzyv6ycmqiutloyvypvmagn7xvqkce@d4xuv3mylpg4

src/include/storage/procnumber.h

@@ -25,6 +25,19 @@ typedef int ProcNumber;
 
 #define INVALID_PROC_NUMBER		(-1)
 
+/*
+ * Note: MAX_BACKENDS_BITS is 18 as that is the space available for buffer
+ * refcounts in buf_internals.h.  This limitation could be lifted by using a
+ * 64bit state; but it's unlikely to be worthwhile as 2^18-1 backends exceed
+ * currently realistic configurations. Even if that limitation were removed,
+ * we still could not a) exceed 2^23-1 because inval.c stores the ProcNumber
+ * as a 3-byte signed integer, b) INT_MAX/4 because some places compute
+ * 4*MaxBackends without any overflow check.  This is rechecked in the
+ * relevant GUC check hooks and in RegisterBackgroundWorker().
+ */
+#define MAX_BACKENDS_BITS		18
+#define MAX_BACKENDS			((1U << MAX_BACKENDS_BITS)-1)
+
 /*
  * Proc number of this backend (same as GetNumberFromPGProc(MyProc))
  */