Improve radix point (decimal point/comma) handling

The GETDECIMAL(x) macro in src/cmd/ksh93/features/locale uses a
global static variable 'lp' to get the localeconv() results.
Several functions in ksh use local variables called 'lp'. It's dumb
luck that this hasn't conflicted yet; it's a bug waiting to happen.
It's also slightly inefficient as it calls localeconv() every time.

In addition there is a redundant 'sh.decomma' flag that is set to 1
if the radix point is a comma, but only once, by sh_init(). It is
not updated if the locale changes. That is not correct.

This commit gets rid of both and implements a new approach instead:
store the radix point in sh.radixpoint at init time in sh_init(),
and in the put_lang() locale discipline function, reinitialise
sh.radixpoint whenever LC_ALL or LC_NUMERIC changes. The rest of
the code can then simply use sh.radixpoint without worry.

src/cmd/ksh93/include/shell.h

@@ -322,7 +322,7 @@ struct Shell_s
 	char		subshare;	/* set when comsub==2 (shared-state ${ ...; } command substitution) */
 	char		toomany;	/* set when out of fd's */
 	char		instance;	/* in set_instance */
-	char		decomma;	/* decimal_point=',' */
+	char		radixpoint;	/* current radix point ('.' or ',') */
 	char		redir0;		/* redirect of 0 */
 	char		intrace;	/* set when trace expands PS4 */
 	char		*readscript;	/* set before reading a script */