top: reorganize some fields management logic

The calibrate_fields function had grown too large and
was adversly impacting maintainence.  So half of the
logic was split out into a new function.

Now, maintainence of column headers and the required
library flags is organized as follows:

 . adj_geometry   (calibrate_fields helper)
     provides low-level support for sigwinch, memory

 . build_headers  (calibrate_fields helper)
     constructs the headers and library flags

 . calibrate_fields
     establishes which fields will be displayed

( note the alpha order mentioned in a prior commit )

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

top/top.h

@@ -568,6 +568,7 @@ typedef struct WIN_t {
 /*------  Fields Management support  -------------------------------------*/
 /*atic FLD_t         Fieldstab[] = { ... }                                */
 //atic void          adj_geometry (void);
+//atic void          build_headers (void);
 //atic void          calibrate_fields (void);
 //atic void          display_fields (int focus, int extend);
 //atic void          fields_utility (void);