Clean up weird corner cases in lexing of psql meta-command arguments.

These changes allow backtick command evaluation and psql variable
interpolation to happen on substrings of a single meta-command argument.
Formerly, no such evaluations happened at all if the backtick or colon
wasn't the first character of the argument, and we considered an argument
completed as soon as we'd processed one backtick, variable reference, or
quoted substring.  A string like 'FOO'BAR was thus taken as two arguments
not one, not exactly what one would expect.  In the new coding, an argument
is considered terminated only by unquoted whitespace or backslash.

Also, clean up a bunch of omissions, infelicities and outright errors in
the psql documentation of variables and metacommand argument syntax.

src/bin/psql/command.c

@@ -121,7 +121,7 @@ HandleSlashCmds(PsqlScanState scan_state,
 		/* eat any remaining arguments after a valid command */
 		/* note we suppress evaluation of backticks here */
 		while ((arg = psql_scan_slash_option(scan_state,
-											 OT_VERBATIM, NULL, false)))
+											 OT_NO_EVAL, NULL, false)))
 		{
 			psql_error("\\%s: extra argument \"%s\" ignored\n", cmd, arg);
 			free(arg);

src/bin/psql/psqlscan.h

@@ -33,7 +33,7 @@ enum slash_option_type
 	OT_SQLIDHACK,				/* SQL identifier, but don't downcase */
 	OT_FILEPIPE,				/* it's a filename or pipe */
 	OT_WHOLE_LINE,				/* just snarf the rest of the line */
-	OT_VERBATIM					/* literal (no backticks or variables) */
+	OT_NO_EVAL					/* no expansion of backticks or variables */
 };