lib/locale.t: Only output debug message if relevant

This message telling people to set an environment variable was output
even if the variable was already set; this is potentially confusing
utf8.h: Fix grammar in comment
perlapi: Nits
perluniprops: Nit

This uses the singular, but might be output applying to more than one.
mktables: Default those tables whose names begin with '_' to internal

Some tables that mktables generates are for specialized internal core
use, and should not be accessible through Unicode::UCD nor listed in
perluniprops.  The creation code for _Perl_Any_Folds was moved by commit
8d88896d10ee81fb95a9494ca771f9d2b14f5fc7, but it wasn't a clean move and
the line that caused it to be marked as internal was accidentally removed.
This commit changes things to default to internal-only if the name
begins with an underscore, overridable by an explicit setting.  Thus the
line that got omitted is no longer necessary, and future mistakes like
this won't have any effect.  This is the only property currently
affected by this change.
mktables: Clean up relict usage

The 'status' of a table generated by mktables was split into two
concepts; these lines should have been changed to reflect that split,
but weren't.  This didn't affect what actually gets generated.
concept
PATCH: [perl #120675] Unexpected tainting via regex using locale

Tainting should only be turned on if something that could be tainted is
actually tried.  Hence in a [bracketed character class], if the
character being matched is something like

    "a" =~ /([a-z])/l;

tainting isn't turned on because the match doesn't actually depend on
the locale definition.  That is, it would match regardless of the
locale.  Similarly,

    "A" =~ /([a-z])/l;

shouldn't turn on tainting because it shouldn't match no matter what
locale is in effect.  However, until this commit, this worked only
because of a compilte time optimization.  The following (from the
ticket) isn't optimized and didn't work until this commit:

    "foo.bar_baz" =~ /^(.*)[._](.*?)$/';

The [._] was causing tainting even though there is nothing in the
pattern that is locale-dependent.

This commit just moves the tainting to code branches where the locale
actually does matter.  It was just turning it on for any bracketed
character class where the target character didn't match any of the
explicit characters in the class.
perllocale: Nit, and clarification
perllocale: Add info about locale testing.