library: normalize the readstat context structure name

The new library meminfo & vmstat modules use structure
names for their context which exactly mirror the names
of the very /proc/ files whose particulars they yield.

The one exception to this rule was the readstat module
whose struct was named statinfo yet the file was stat.

This commit simply renames that structure (only) so as
to hopefully establish such a naming convention as our
standard going forward. And, it's makes good symmetry.

[ this module's name itself is just perfect as it is ]

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

ps/output.c

@@ -98,7 +98,7 @@ extern long Hertz;
 
 static void get_boot_time(void)
 {
-    struct procps_statinfo *sys_info;
+    struct procps_stat *sys_info;
     if (procps_stat_new(&sys_info) < 0)
 	xerrx(EXIT_FAILURE,
 		_("Unable to create system stat structure"));