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.

contrib/file_fdw/file_fdw.c

@@ -588,6 +588,7 @@ fileBeginForeignScan(ForeignScanState *node, int eflags)
 	 */
 	cstate = BeginCopyFrom(node->ss.ss_currentRelation,
 						   filename,
+						   false,
 						   NIL,
 						   options);
 
@@ -660,6 +661,7 @@ fileReScanForeignScan(ForeignScanState *node)
 
 	festate->cstate = BeginCopyFrom(node->ss.ss_currentRelation,
 									festate->filename,
+									false,
 									NIL,
 									festate->options);
 }
@@ -993,7 +995,7 @@ file_acquire_sample_rows(Relation onerel, int elevel,
 	/*
 	 * Create CopyState from FDW options.
 	 */
-	cstate = BeginCopyFrom(onerel, filename, NIL, options);
+	cstate = BeginCopyFrom(onerel, filename, false, NIL, options);
 
 	/*
 	 * Use per-tuple memory context to prevent leak of memory used to read