Add help & tab-complete support for psql's \getenv.

I forgot about these details in 33d3eeadb :-(.
Noted by Christoph Berg.

Discussion: https://postgr.es/m/YcI8i/mduMi91uXY@msg.df7cb.de

src/bin/psql/help.c

@@ -166,7 +166,7 @@ slashUsage(unsigned short int pager)
 	 * Use "psql --help=commands | wc" to count correctly.  It's okay to count
 	 * the USE_READLINE line even in builds without that.
 	 */
-	output = PageOutput(136, pager ? &(pset.popt.topt) : NULL);
+	output = PageOutput(137, pager ? &(pset.popt.topt) : NULL);
 
 	fprintf(output, _("General\n"));
 	fprintf(output, _("  \\copyright             show PostgreSQL usage and distribution terms\n"));
@@ -309,6 +309,7 @@ slashUsage(unsigned short int pager)
 
 	fprintf(output, _("Operating System\n"));
 	fprintf(output, _("  \\cd [DIR]              change the current working directory\n"));
+	fprintf(output, _("  \\getenv PSQLVAR ENVVAR fetch environment variable\n"));
 	fprintf(output, _("  \\setenv NAME [VALUE]   set or unset environment variable\n"));
 	fprintf(output, _("  \\timing [on|off]       toggle timing of commands (currently %s)\n"),
 			ON(pset.timing));

src/bin/psql/tab-complete.c

@@ -1519,7 +1519,7 @@ psql_completion(const char *text, int start, int end)
 		"\\echo", "\\edit", "\\ef", "\\elif", "\\else", "\\encoding",
 		"\\endif", "\\errverbose", "\\ev",
 		"\\f",
-		"\\g", "\\gdesc", "\\gexec", "\\gset", "\\gx",
+		"\\g", "\\gdesc", "\\getenv", "\\gexec", "\\gset", "\\gx",
 		"\\help", "\\html",
 		"\\if", "\\include", "\\include_relative", "\\ir",
 		"\\list", "\\lo_import", "\\lo_export", "\\lo_list", "\\lo_unlink",