patch 8.1.2225: the "last used" info of a buffer is under used

Problem:    The "last used" info of a buffer is under used.
Solution:   Add "lastused" to getbufinfo(). List buffers sorted by last-used
            field. (Andi Massimino, closes #4722)

runtime/doc/eval.txt

@@ -4778,6 +4778,10 @@ getbufinfo([{dict}])
 			changed		TRUE if the buffer is modified.
 			changedtick	number of changes made to the buffer.
 			hidden		TRUE if the buffer is hidden.
+			lastused	timestamp in seconds, like
+					|localtime()|, when the buffer was
+					last used.
+					{only with the |+viminfo| feature}
 			listed		TRUE if the buffer is listed.
 			lnum		current line number in buffer.
 			loaded		TRUE if the buffer is loaded.

runtime/doc/options.txt

@@ -8689,6 +8689,8 @@ A jump table for the options with a short description can be found at |Q_op|.
 			complete first match.
 	"list:longest"	When more than one match, list all matches and
 			complete till longest common string.
+	"list:lastused" When more than one buffer matches, sort buffers
+			by time last used (other than the current buffer).
 	When there is only a single match, it is fully completed in all cases.
 
 	Examples: >

runtime/doc/windows.txt

@@ -1090,6 +1090,7 @@ list of buffers. |unlisted-buffer|
 		     R	 terminal buffers with a running job
 		     F	 terminal buffers with a finished job
 		     ?   terminal buffers without a job: `:terminal NONE`
+		     t   show time last used and sort buffers
 		Combining flags means they are "and"ed together, e.g.:
 		     h+   hidden buffers which are modified
 		     a+   active buffers which are modified