[count] Allow counting lines from stdin

As a simple replacement for `wc -l`.

This counts both lines on stdin _and_ arguments.

So if "file" has three lines, then `count a b c < file` will print 6.

And since it counts newlines, like wc, `echo -n foo | count` prints 0.

tests/count.err

@@ -13,3 +13,6 @@
 
 ####################
 # big counts
+
+####################
+# stdin

tests/count.in

@@ -27,3 +27,12 @@ for i in seq 500
         break
     end
 end
+
+logmsg stdin
+# Reading from stdin still counts the arguments
+printf '%s\n' 1 2 3 4 5 | count 6 7 8 9 10
+
+# Reading from stdin counts newlines - like `wc -l`.
+echo -n 0 | count
+
+echo 1 | count

tests/count.out

@@ -21,3 +21,9 @@
 
 ####################
 # big counts
+
+####################
+# stdin
+10
+0
+1