rewrite core of the glob implementation for correctness & optimization

this code has been long overdue for a rewrite, but the immediate cause
that necessitated it was total failure to see past unreadable path
components. for example, A/B/* would fail to match anything, even
though it should succeed, when both A and A/B are searchable but only
A/B is readable. this problem both was caught in conformance testing,
and impacted users.

the old glob implementation insisted on searching the listing of each
path component for a match, even if the next component was a literal.
it also used considerable stack space, up to length of the pattern,
per recursion level, and relied on an artificial bound of the pattern
length by PATH_MAX, which was incorrect because a pattern can be much
longer than PATH_MAX while having matches shorter (for example, with
necessarily long bracket expressions, or with redundancy).

in the new implementation, each level of recursion starts by consuming
the maximal literal (possibly escaped-literal) path prefix remaining
in the pattern, and only opening a directory to read when there is a
proper glob pattern in the next path component. it then recurses into
each matching entry. the top-level glob function provided automatic
storage (up to PATH_MAX) for construction of candidate/result strings,
and allocates a duplicate of the pattern that can be modified in-place
with temporary null-termination to pass to fnmatch. this allocation is
not a big deal since glob already has to perform allocation, and has
to link free to clean up if it experiences an allocation failure or
other error after some results have already been allocated.

care is taken to use the d_type field from iterated dirents when
possible; stat is called only when there are literal path components
past the last proper-glob component, or when needed to disambiguate
symlinks for the purpose of GLOB_MARK.

one peculiarity with the new implementation is the manner in which the
error handling callback will be called. if attempting to match */B/C/D
where a directory A exists that is inaccessible, the error reported
will be a stat error for A/B/C/D rather than (previous and wrong
implementation) an opendir error for A, or (likely on other
implementations) a stat error for A/B. such behavior does not seem to
be non-conforming, but if it turns out to be undesirable for any
reason, backtracking could be done on error to report the first
component producing it.

also, redundant slashes are no longer normalized, but preserved as
they appear in the pattern; this is probably more correct, and falls
out naturally from the algorithm used. since trailing slashes (which
force all matches to be directories) are preserved as well, the
behavior of GLOB_MARK has been adjusted not to append an additional
slash to results that already end in slash.

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