Add support for piping COPY to/from an external program.

This includes backend "COPY TO/FROM PROGRAM '...'" syntax, and corresponding
psql \copy syntax. Like with reading/writing files, the backend version is
superuser-only, and in the psql version, the program is run in the client.

In the passing, the psql \copy STDIN/STDOUT syntax is subtly changed: if you
the stdin/stdout is quoted, it's now interpreted as a filename. For example,
"\copy foo from 'stdin'" now reads from a file called 'stdin', not from
standard input. Before this, there was no way to specify a filename called
stdin, stdout, pstdin or pstdout.

This creates a new function in pgport, wait_result_to_str(), which can
be used to convert the exit status of a process, as returned by wait(3),
to a human-readable string.

Etsuro Fujita, reviewed by Amit Kapila.

src/include/nodes/parsenodes.h

@@ -1407,6 +1407,7 @@ typedef struct CopyStmt
 	List	   *attlist;		/* List of column names (as Strings), or NIL
 								 * for all columns */
 	bool		is_from;		/* TO or FROM */
+	bool		is_program;		/* is 'filename' a program to popen? */
 	char	   *filename;		/* filename, or NULL for STDIN/STDOUT */
 	List	   *options;		/* List of DefElem nodes */
 } CopyStmt;