degesch: allow launching an editor for input

Useful for editing multiline text (such as making it single-line).

Some refactoring and cleanup.

common.c

@@ -54,6 +54,49 @@ str_vector_find (const struct str_vector *v, const char *s)
 	return -1;
 }
 
+/// This differs from the non-unique version in that we expect the filename
+/// to be something like a pattern for mkstemp(), so the resulting path can
+/// reside in a system-wide directory with no risk of a conflict.
+static char *
+resolve_relative_runtime_unique_filename (const char *filename)
+{
+	struct str path;
+	str_init (&path);
+
+	const char *runtime_dir = getenv ("XDG_RUNTIME_DIR");
+	if (runtime_dir && *runtime_dir == '/')
+		str_append (&path, runtime_dir);
+	else
+		str_append (&path, "/tmp");
+	str_append_printf (&path, "/%s/%s", PROGRAM_NAME, filename);
+
+	// Try to create the file's ancestors;
+	// typically the user will want to immediately create a file in there
+	const char *last_slash = strrchr (path.str, '/');
+	if (last_slash && last_slash != path.str)
+	{
+		char *copy = xstrndup (path.str, last_slash - path.str);
+		(void) mkdir_with_parents (copy, NULL);
+		free (copy);
+	}
+	return str_steal (&path);
+}
+
+static bool
+xwrite (int fd, const char *data, size_t len, struct error **e)
+{
+	size_t written = 0;
+	while (written < len)
+	{
+		ssize_t res = write (fd, data + written, len - written);
+		if (res >= 0)
+			written += res;
+		else if (errno != EINTR)
+			FAIL ("%s", strerror (errno));
+	}
+	return true;
+}
+
 // --- Logging -----------------------------------------------------------------
 
 static void