main: add d_reclen as an alias for d_namelen (struct dirent)

Though protable-dirent.h defineds d_namelen as a field of
struct dirent, protable-scan.c expects it as d_reclen.

man page of getdents in GNU/Linux calls it d_reclen.
This commit follows the naming in the man page.

Signed-off-by: Masatake YAMATO <yamato@redhat.com>

main/portable-dirent.h

@@ -166,6 +166,7 @@ extern "C" {
 typedef struct dirent
 {
    char d_name[MAX_PATH + 1];                  /* File name */
+#define d_reclen d_namelen
    size_t d_namlen;                            /* Length of name without \0 */
    int d_type;                                 /* File type */
 } dirent;