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/bin/psql/stringutils.h

@@ -19,6 +19,8 @@ extern char *strtokx(const char *s,
 		bool del_quotes,
 		int encoding);
 
+extern void strip_quotes(char *source, char quote, char escape, int encoding);
+
 extern char *quote_if_needed(const char *source, const char *entails_quote,
 				char quote, char escape, int encoding);