set env var NSXIV_USING_NULL for key-handler and update docs

Co-authored-by: Berke Kocaoğlu <berke.kocaoglu@metu.edu.tr>

examples/key-handler

@@ -3,10 +3,14 @@
 # Example for $XDG_CONFIG_HOME/nsxiv/exec/key-handler
 # Called by nsxiv(1) after the external prefix key (C-x by default) is pressed.
 # The next key combo is passed as its first argument. Passed via stdin are the
-# images to act upon, one path per line: all marked images, if in thumbnail
+# images to act upon: all marked images, if in thumbnail mode and at least one
-# mode and at least one image has been marked, otherwise the current image.
+# image has been marked, otherwise the current image. nsxiv(1) will block until
-# nsxiv(1) blocks until this script terminates. It then checks which images
+# the handler terminates. It then checks which images have been modified and
-# have been modified and reloads them.
+# reloads them.
+
+# By default nsxiv(1) will send one image per-line to stdin, however when using
+# -0 the image list will be NULL separated and the enviornment variable
+# "$NSXIV_USING_NULL" will be set to 1.
 
 # The key combo argument has the following form: "[C-][M-][S-]KEY",
 # where C/M/S indicate Ctrl/Meta(Alt)/Shift modifier states and KEY is the X

main.c

@@ -549,6 +549,7 @@ void run_key_handler(const char *key, unsigned int mask)
 	         mask & ControlMask ? "C-" : "",
 	         mask & Mod1Mask    ? "M-" : "",
 	         mask & ShiftMask   ? "S-" : "", key);
+	setenv("NSXIV_USING_NULL", options->using_null ? "1" : "0", 1);
 
 	if ((pid = fork()) == 0) {
 		close(pfd[1]);

nsxiv.1

@@ -439,14 +439,20 @@ located in
 The handler is invoked by pressing
 .BR Ctrl-x .
 The next key combo is passed as its first argument. Passed via stdin are the
-images to act upon, one path per line: all marked images, if in thumbnail mode
+images to act upon: all marked images, if in thumbnail mode and at least one
-and at least one image has been marked, otherwise the current image.
+image has been marked, otherwise the current image. nsxiv(1) will block until
-nsxiv(1) will block until the handler terminates. It then checks which images
+the handler terminates. It then checks which images have been modified and
-have been modified and reloads them.
+reloads them.
+
+By default nsxiv(1) will send one image per-line to stdin, however when using
+\-0 the image list will be NULL separated and the enviornment variable
+"$NSXIV_USING_NULL" will be set to 1.
 
 The key combo argument has the following form: "[C-][M-][S-]KEY",
 where C/M/S indicate Ctrl/Meta(Alt)/Shift modifier states and KEY is the X
 keysym as listed in /usr/include/X11/keysymdef.h without the "XK_" prefix.
+If KEY has an uppercase equivalent, S-KEY is resolved into it. For instance,
+K replaces S-k and Scedilla replaces S-scedilla, but S-Delete is sent as-is.
 
 There is also an example script installed together with nsxiv as
 .IR EGPREFIX/key-handler .