Prefer UNIX_PATH_MAX over UNIX_PATH_LEN

Cygwin used to define UNIX_PATH_LEN, but in cygwin-1_7_18-release and
cygwin-64bit-branch, sys/un.h renamed to MAX to follow Linux. (CVS r1.7)

This commit will be pushed using 64-bit ssh-pageant, proving that all is
well here for cygwin64.  For the adventurous, it also does work to let
cygwin64 processes talk to cygwin32 ssh-pageant, and vice versa, as long
as you reconcile their different filesystem mounts in SSH_AUTH_SOCK.

main.c

@@ -28,6 +28,12 @@
 #include "winpgntc.h"
 
 
+// Cygwin now has UNIX_PATH_MAX, but used to be _LEN
+#ifndef UNIX_PATH_MAX
+#define UNIX_PATH_MAX UNIX_PATH_LEN
+#endif
+
+
 #define FD_FOREACH(fd, set) \
     for (fd = 0; fd < FD_SETSIZE; ++fd) \
         if (FD_ISSET(fd, set))
@@ -39,8 +45,8 @@ struct fd_buf {
 };
 
 
-static char cleanup_tempdir[UNIX_PATH_LEN] = "";
-static char cleanup_sockpath[UNIX_PATH_LEN] = "";
+static char cleanup_tempdir[UNIX_PATH_MAX] = "";
+static char cleanup_sockpath[UNIX_PATH_MAX] = "";
 
 
 static void cleanup_exit(int status) __attribute__((noreturn));
@@ -292,7 +298,7 @@ do_agent_loop(int sockfd)
 int
 main(int argc, char *argv[])
 {
-    char sockpath[UNIX_PATH_LEN] = "";
+    char sockpath[UNIX_PATH_MAX] = "";
     static struct option long_options[] = {
         { "help", no_argument, 0, 'h' },
         { "version", no_argument, 0, 'v' },