sv.h: Add comment
Properly determine if in locale for NV stringification

Commit 68e8f474bc686a86c064b695b9c7400313d7af65 used the wrong macro to
determine if the operation is within the scope of 'use locale'.  It also
has to operate under 'use locale 'not_characters'.

Tests for this are coming in the patch for [perl #118197].
PATCH: [perl #118197] Cope with non-ASCII decimal separators

This patch causes the radix string to be examined upon a new numeric
locale being set.  If the string isn't ASCII, and the new locale is
UTF-8, it turns on the UTF-8 flag in the scalar that holds the radix.
When a floating point number is formatted in Perl_sv_vcatpvfn_flags(),
and the flag is on, the result's flag will be set on too.
util.c: Avoid unnecessary setlocale() calls

This code sets the locale to C around its work.  This is unnecessary if
the locale is already C, and there is an existing global that indicates
this, that can be tested to avoid the sets.
pp_sys.c: Use macro instead of reinventing it

The Strerror macro is defined properly to handle either case here.
perl.c: Use Strerror instead of strerror

The former works on more systems.