add -0 for outputting null-terminated list (#68)

* add -0 for outputting null-terminated list

this doesn't add much, if any, additional complexity to the codebase and
can be quite handy for scripting purposes.

Closes: https://github.com/nsxiv/nsxiv/issues/67

* Fix typo

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

commands.c

@@ -59,7 +59,7 @@ bool cg_quit(arg_t _)
 	if (options->to_stdout && markcnt > 0) {
 		for (i = 0; i < filecnt; i++) {
 			if (files[i].flags & FF_MARK)
-				printf("%s\n", files[i].name);
+				printf("%s%c", files[i].name, options->stdout_separator);
 		}
 	}
 	exit(EXIT_SUCCESS);

nsxiv.h

@@ -268,6 +268,7 @@ struct opt {
 	char **filenames;
 	bool from_stdin;
 	bool to_stdout;
+	char stdout_separator;
 	bool recursive;
 	int filecnt;
 	int startnum;

options.c

@@ -52,6 +52,7 @@ void parse_options(int argc, char **argv)
 
 	_options.from_stdin = false;
 	_options.to_stdout = false;
+	_options.stdout_separator = '\n';
 	_options.recursive = false;
 	_options.startnum = 0;
 
@@ -75,7 +76,7 @@ void parse_options(int argc, char **argv)
 	_options.clean_cache = false;
 	_options.private_mode = false;
 
-	while ((opt = getopt(argc, argv, "A:abce:fG:g:hin:N:opqrS:s:T:tvZz:")) != -1) {
+	while ((opt = getopt(argc, argv, "A:abce:fG:g:hin:N:0opqrS:s:T:tvZz:")) != -1) {
 		switch (opt) {
 			case '?':
 				print_usage();
@@ -128,6 +129,10 @@ void parse_options(int argc, char **argv)
 			case 'N':
 				_options.res_name = optarg;
 				break;
+			case '0':
+				_options.stdout_separator = '\0';
+				/* -0 implies -o */
+				/* fall through */
 			case 'o':
 				_options.to_stdout = true;
 				break;