t/lib-httpd: switch SANITY check for NOT_ROOT

t/lib-httpd: switch SANITY check for NOT_ROOT

The SANITY prerequisite is really about whether the
filesystem will respect the permissions we set, and being
root is only one part of that. But the httpd tests really
just care about not being root, as they are trying to avoid
weirdness in apache (see a1a3011 for details).

Let's switch out SANITY for a new NOT_ROOT prerequisite,
which will let us tweak SANITY more freely.

We implement NOT_ROOT by checking `id -u`, which is in POSIX
and seems to be available even on MSYS.  Note that we cannot
just call this "ROOT" and ask for "!ROOT". The possible
outcomes are:

  1. we know we are root

  2. we know we are not root

  3. we could not tell, because `id` was not available

We should conservatively treat (3) as "does not have the
prerequisite", which means that a naive negation would not
work.

Helped-by: Kyle J. McKay <mackyle@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>