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/sysinfo.h

@@ -9,6 +9,7 @@ EXTERN_C_BEGIN
 extern unsigned long long Hertz;   /* clock tick frequency */
 extern long smp_num_cpus;          /* number of CPUs */
 extern int have_privs;             /* boolean, true if setuid or similar */
+extern long page_bytes;            /* this architecture's bytes per page */
 
 #if 0
 #define JT double