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/backend/nodes/copyfuncs.c

@@ -2703,6 +2703,7 @@ _copyCopyStmt(const CopyStmt *from)
 	COPY_NODE_FIELD(query);
 	COPY_NODE_FIELD(attlist);
 	COPY_SCALAR_FIELD(is_from);
+	COPY_SCALAR_FIELD(is_program);
 	COPY_STRING_FIELD(filename);
 	COPY_NODE_FIELD(options);
 

src/backend/nodes/equalfuncs.c

@@ -1090,6 +1090,7 @@ _equalCopyStmt(const CopyStmt *a, const CopyStmt *b)
 	COMPARE_NODE_FIELD(query);
 	COMPARE_NODE_FIELD(attlist);
 	COMPARE_SCALAR_FIELD(is_from);
+	COMPARE_SCALAR_FIELD(is_program);
 	COMPARE_STRING_FIELD(filename);
 	COMPARE_NODE_FIELD(options);