library: tweak newly added MemAvailable fall-back code

This commit just ensures recalculation of some amounts
for iterative processes, like top. It also trades some
repeated runtime calls to sysconf for a one time cost.

Reference(s):
http://www.freelists.org/post/procps/systemd-support-to-library,7
. fall-back calculations
commit b779855cf1

Signed-off-by: Jim Warner <james.warner@comcast.net>

proc/libprocps.sym

@@ -44,6 +44,7 @@ global:
 	open_psdb;
 	open_psdb_message;
 	openproc;
+	page_bytes;
 	pretty_print_signals;
 	print_uptime;
 	procps_version;