top: exploit new kb_main_available, make Jaromir happy

This patch will trade a former pessimistic calculation
of free physical memory for a more optimistic one that
uses the newly added kb_main_available library export.

But in case one might wish to return to the old former
method, there's a new #define that was made available.

[ the new calculation will affect graphing mode only ]

Reference(s):
http://www.freelists.org/post/procps/systemd-support-to-library,9

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

top/top.h

@@ -41,6 +41,7 @@
 //#define INSP_OFFDEMO            /* disable demo screens, issue msg instead */
 //#define INSP_SAVEBUF            /* preserve 'Insp_buf' contents in a file  */
 //#define INSP_SLIDE_1            /* when scrolling left/right don't move 8  */
+//#define MEMGRAPH_OLD            /* don't use 'available' when graphing Mem */
 //#define OFF_HST_HASH            /* use BOTH qsort+bsrch vs. hashing scheme */
 //#define OFF_NUMASKIP            /* do NOT skip numa nodes if discontinuous */
 //#define OFF_SCROLLBK            /* disable tty emulators scrollback buffer */
@@ -658,6 +659,9 @@ typedef struct WIN_t {
 #if defined(TERMIOS_ONLY)
 # warning 'TERMIOS_ONLY' disables input recall and makes man doc incorrect
 #endif
+#if defined(MEMGRAPH_OLD)
+# warning 'MEMGRAPH_OLD' will make the man document Section 2c. misleading
+#endif
 
 
 /*######  Some Prototypes (ha!)  #########################################*/