Fix import of float attribute/value from environment (re: 960a1a99)

Bug 1: as of 960a1a99, floating point literals were no longer
recognised when importing variables from the environment. The
attribute was still imported but the value reverted to zero:

$ (export LC_NUMERIC=C; typeset -xF5 num=7.75; \
   ksh -c 'typeset -p num')
typeset -x -F 5 num=0.00000

Bug 2 (inherited from 93u+): The code that imported variable
attributes from the environment only checked '.' to determine
whether the float attribute should be set. It should check the
current radix point instead.

$ (export LC_NUMERIC=debug; typeset -xF5 num=7,75; \
   ksh -c 'typeset -p num')
typeset -x -i num=0
...or, after fixing bug 1 only, the output is:
typeset -x -i num=75000

src/cmd/ksh93/sh/arith.c: sh_strnum():
- When importing untrusted env vars at init time, handle not only
  "base#value" literals using strtonll, but also floating point
  literals using strtold. This fixes the bug without reallowing
  arbitary expressions. (re: 960a1a99)
- When not initialising, use sh.radixpoint (see f0386a87) instead
  of '.' to help decide whether to evaluate an arith expression.

src/cmd/ksh93/sh/init.c: env_import_attributes():
- Use sh.radixpoint instead of '.' to check for a decimal fraction.
  (This code is needed because doubles are exported as integers for
  ksh88 compatibility; see attstore() in name.c.)

src/cmd/ksh93/sh/init.c

@@ -2089,17 +2089,17 @@ static void env_import_attributes(char *next)
 				/* check for floating */
 				char *dp, *val = nv_getval(np);
 				strtol(val,&dp,10);
-				if(*dp=='.' || *dp=='e' || *dp=='E')
+				if(*dp==sh.radixpoint || *dp=='e' || *dp=='E')
 				{
 					char *lp;
 					flag |= NV_DOUBLE;
-					if(*dp=='.')
+					if(*dp==sh.radixpoint)
 					{
 						strtol(dp+1,&lp,10);
 						if(*lp)
 							dp = lp;
 					}
-					if(*dp && *dp!='.')
+					if(*dp && *dp!=sh.radixpoint)
 					{
 						flag |= NV_EXPNOTE;
 						size = dp-val;