library: properly handle memory used by tmpfs

tmpfs has become much more widely used since distributions use it for
/tmp (Fedora 18+). In /proc/meminfo, memory used by tmpfs is accounted
into "Cached" (aka "NR_FILE_PAGES",
 http://lxr.free-electrons.com/source/mm/shmem.c#L301 ).

The tools just pass it on, so what top, free and vmstat report as
"cached" is the sum of page cache and tmpfs.

free has the extremely useful "-/+ buffers/cache" output. However, now
that tmpfs is accounted into "cached", those numbers are way off once
you have big files in /tmp.

Fortunately, kernel 2.6.32 introduces "Shmem", which makes tmpfs memory
usage accessible from userspace (
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=4b02108ac1b3354a22b0d83c684797692efdc395 ).

This patch substracts Shmem from Cached to get the actual page cache
memory. This makes both issues mentioned above disappear. For older
kernels, Shmem is not available (hence zero) and this patch is no-op.

Additionally:
* Update the man pages of free and vmstat to explain what is happening
* Finally drop "MemShared" from the /proc/meminfo parser, it has been
  dead for 10+ years and is only causing confusion ( removed in kernel
  2.5.54, see
  https://git.kernel.org/cgit/linux/kernel/git/tglx/history.git/commit/?id=fe04e9451e5a159247cf9f03c615a4273ac0c571 )

proc/sysinfo.h

@@ -20,8 +20,7 @@ extern int        uptime (double *uptime_secs, double *idle_secs);
 extern unsigned long getbtime(void);
 extern void       loadavg(double *av1, double *av5, double *av15);
 
-
-/* obsolete */
+/* Shmem in 2.6.32+ */
 extern unsigned long kb_main_shared;
 /* old but still kicking -- the important stuff */
 extern unsigned long kb_main_buffers;