The '[[:digit:]]' and '[[:alpha:]]' classes require double brackets, not
single.

The regex functions return nonzero (not necessarily negative values) on
failure.

In order to avoid us being unable to match characters which are part of
the normal US alphabet in certain weird languages, add two tests to
catch this behavior.

In order to make it easier adding more locale-related tests, add a
generalized framework handling initial setup of languages as well as the
cleanup of them afterwards.

While we already have a test for `p_regexec` with `LC_CTYPE` being
modified, `regexec` also alters behavior as soon as `LC_COLLATE` is
being modified. Most importantly, `LC_COLLATE` changes the way how
ranges are interpreted to just not handling them at all. Thus, ensure
that either we use `regcomp_l` to avoid this, or that we've fallen back
to our builtin regex functionality which also behaves properly.

While the test asserts that the error value indcates a non-value, it is
actually never getting assigned to. Fix this.

Prefix all the calls to the the regexec family of functions with `p_`.
This allows us to swap out all the regular expression functions with our
own implementation.  Move the declarations to `posix_regex.h` for
simpler inclusion.

Test that largefiles can be read through the tree API