library: refactor and rely on modern kernels for wchan

Several Debian based distributions were recently found
to have omitted a kernel configuration option that had
the effect of rendering /proc/#/stat and /proc/#/wchan
useless for providing any 'sleeping in function' info.

That problem also prompted a reevaluation of the whole
approach to wchan matters which had grown increasingly
complex as our library evolved over the last 13 years.

The net result was a decision to rely on /proc/#/wchan
which arrived along with the 2.5 kernel. This then let
us vastly simplify the internal code plus the external
interface which will benefit both the top and ps pgms.

Reference(s):
http://www.freelists.org/post/procps/WCHAN,11
https://lkml.org/lkml/2008/11/6/12
https://bugs.debian.org/711592

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

proc/readproc.h

@@ -287,7 +287,6 @@ extern proc_t * get_proc_stats(pid_t pid, proc_t *p);
 #define PROC_FILLGRP         0x0010 // resolve group id number -> group name
 #define PROC_FILLSTATUS      0x0020 // read status
 #define PROC_FILLSTAT        0x0040 // read stat
-#define PROC_FILLWCHAN       0x0080 // look up WCHAN name
 #define PROC_FILLARG         0x0100 // alloc and fill in `cmdline'
 #define PROC_FILLCGROUP      0x0200 // alloc and fill in `cgroup`
 #define PROC_FILLSUPGRP      0x0400 // resolve supplementary group id -> group name