Add d_type to our Windows dirent emulation.

This allows us to skip some stat calls, by extending commit 861c6e7c to
cover Windows systems.

Author: Juan José Santamaría Flecha <juanjo.santamaria@gmail.com>
Reviewed-by: Alvaro Herrera <alvherre@2ndquadrant.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Magnus Hagander <magnus@hagander.net>
Reviewed-by: Thomas Munro <thomas.munro@gmail.com>
Discussion: https://postgr.es/m/CA%2BhUKG%2BFzxupGGN4GpUdbzZN%2Btn6FQPHo8w0Q%2BAPH5Wz8RG%2Bww%40mail.gmail.com

src/include/port/win32_msvc/dirent.h

@@ -10,6 +10,7 @@ struct dirent
 {
 	long		d_ino;
 	unsigned short d_reclen;
+	unsigned char d_type;
 	unsigned short d_namlen;
 	char		d_name[MAX_PATH];
 };
@@ -20,4 +21,14 @@ DIR		   *opendir(const char *);
 struct dirent *readdir(DIR *);
 int			closedir(DIR *);
 
+/* File types for 'd_type'.  */
+#define DT_UNKNOWN		0
+#define DT_FIFO		1
+#define DT_CHR			2
+#define DT_DIR			4
+#define DT_BLK			6
+#define DT_REG			8
+#define DT_LNK			10
+#define DT_SOCK		12
+#define DT_WHT			14
 #endif

src/port/dirent.c

@@ -69,6 +69,7 @@ opendir(const char *dirname)
 	d->handle = INVALID_HANDLE_VALUE;
 	d->ret.d_ino = 0;			/* no inodes on win32 */
 	d->ret.d_reclen = 0;		/* not used on win32 */
+	d->ret.d_type = DT_UNKNOWN;
 
 	return d;
 }
@@ -105,6 +106,15 @@ readdir(DIR *d)
 	}
 	strcpy(d->ret.d_name, fd.cFileName);	/* Both strings are MAX_PATH long */
 	d->ret.d_namlen = strlen(d->ret.d_name);
+	/* The only identified types are: directory, regular file or symbolic link */
+	if ((fd.dwFileAttributes & FILE_ATTRIBUTE_DIRECTORY) != 0)
+		d->ret.d_type = DT_DIR;
+	/* For reparse points dwReserved0 field will contain the ReparseTag */
+	else if ((fd.dwFileAttributes & FILE_ATTRIBUTE_REPARSE_POINT) != 0 &&
+			 (fd.dwReserved0 == IO_REPARSE_TAG_MOUNT_POINT))
+		d->ret.d_type = DT_LNK;
+	else
+		d->ret.d_type = DT_REG;
 
 	return &d->ret;
 }