Add bt_multi_page_stats() function to contrib/pageinspect.

This is like the existing bt_page_stats() function, but it can
report on a range of pages rather than just one at a time.

I don't have a huge amount of faith in the portability of the
new test cases, but they do pass in a 32-bit FreeBSD VM here.
Further adjustment may be needed depending on buildfarm results.

Hamid Akhtar, reviewed by Naeem Akhter, Bertrand Drouvot,
Bharath Rupireddy, and myself

Discussion: https://postgr.es/m/CANugjht-=oGMRmNJKMqnBC69y7vr+wHDmm0ZK6-1pJsxoBKBbA@mail.gmail.com

Author: tglsfdc

contrib/pageinspect/Makefile

@@ -13,7 +13,7 @@ OBJS = \
 	rawpage.o
 
 EXTENSION = pageinspect
-DATA =  pageinspect--1.10--1.11.sql \
+DATA =  pageinspect--1.11--1.12.sql pageinspect--1.10--1.11.sql \
 	pageinspect--1.9--1.10.sql pageinspect--1.8--1.9.sql \
 	pageinspect--1.7--1.8.sql pageinspect--1.6--1.7.sql \
 	pageinspect--1.5.sql pageinspect--1.5--1.6.sql \