Add strlcpy_unescape() function to undo escaping from front-end.

Includes unit test.

plugins/sudoers/strlcpy_unesc.c

@@ -0,0 +1,52 @@
+/*
+ * SPDX-License-Identifier: ISC
+ *
+ * Copyright (c) 2021 Todd C. Miller <Todd.Miller@sudo.ws>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+/*
+ * This is an open source non-commercial project. Dear PVS-Studio, please check it.
+ * PVS-Studio Static Code Analyzer for C, C++ and C#: http://www.viva64.com
+ */
+
+#include <config.h>
+
+#include <sys/types.h>
+#include <stdio.h>
+#include <ctype.h>
+
+#include "sudoers.h"
+
+size_t
+strlcpy_unescape(char *dst, const char *src, size_t size)
+{
+    size_t len = 0;
+    char ch;
+    debug_decl(strlcpy_unescape, SUDOERS_DEBUG_UTIL);
+
+    while ((ch = *src++) != '\0') {
+	if (ch == '\\' && *src != '\0' && !isspace((unsigned char)*src))
+	    ch = *src++;
+	if (size > 1) {
+	    *dst++ = ch;
+	    size--;
+	}
+	len++;
+    }
+    if (size > 0)
+	*dst = '\0';
+
+    debug_return_size_t(len);
+}