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/interfaces/ecpg/preproc/ecpg.addons

@@ -192,7 +192,7 @@ ECPG: where_or_current_clauseWHERECURRENT_POFcursor_name block
 		char *cursor_marker = $4[0] == ':' ? mm_strdup("$0") : $4;
 		$$ = cat_str(2,mm_strdup("where current of"), cursor_marker);
 	}
-ECPG: CopyStmtCOPYopt_binaryqualified_nameopt_column_listopt_oidscopy_fromcopy_file_namecopy_delimiteropt_withcopy_options addon
+ECPG: CopyStmtCOPYopt_binaryqualified_nameopt_column_listopt_oidscopy_fromopt_programcopy_file_namecopy_delimiteropt_withcopy_options addon
 			if (strcmp($6, "from") == 0 &&
 			   (strcmp($7, "stdin") == 0 || strcmp($7, "stdout") == 0))
 				mmerror(PARSE_ERROR, ET_WARNING, "COPY FROM STDIN is not implemented");