Added a configure script to emulate GNU autoconf and related changes.

Changes included in this set:

 * Added strlcat() and strlcpy() from OpenBSD, always use them if the
   system does not have them built in.
 * Changed an array of PATH_MAX size allocated in the stack, to a dynamically
   allocated buffer from heap. This should reduce memory usage a bit.
 * Simplify code that implemented a homegrown realpath(3) implementation,
   simply use realpath(3).
 * If compiler supports -fstack-protector, build all code with
   -D_FORTIFY_SOURCE=2 and --param ssp-buffer-size=1 so that all
   buffers are protected.

bin/xbps-uhelper/Makefile

@@ -1,5 +1,5 @@
 TOPDIR = ../..
-include $(TOPDIR)/vars.mk
+-include $(TOPDIR)/config.mk
 
 BIN = xbps-uhelper
 

bin/xbps-uhelper/main.c

@@ -102,9 +102,9 @@ main(int argc, char **argv)
 {
 	prop_dictionary_t dict;
 	const char *version;
-	char *plist, *pkgname, *pkgver, *in_chroot_env, *root = "";
+	char *plist, *pkgname, *pkgver, *in_chroot_env, *root = "", *hash;
 	bool in_chroot = false;
-	int c, rv = 0;
+	int i, c, rv = 0;
 
 	while ((c = getopt(argc, argv, "Var:")) != -1) {
 		switch (c) {
@@ -299,9 +299,6 @@ main(int argc, char **argv)
 		if (argc < 2)
 			usage();
 
-		char *hash;
-		int i;
-
 		for (i = 1; i < argc; i++) {
 			hash = xbps_get_file_hash(argv[i]);
 			if (hash == NULL) {
@@ -320,7 +317,6 @@ main(int argc, char **argv)
 			usage();
 
 		xbps_fetch_set_cache_connection(0, 0);
-		int i;
 
 		for (i = 1; i < argc; i++) {
 			rv = xbps_fetch_file(argv[i], ".", false, "v");