iconv: Remove alloca use in gconv-modules configuration parsing

The alloca sizes ought to be constrained to PATH_MAX, but replace them
with dynamic allocation to be safe.  A static PATH_MAX array would
have worked too but Hurd does not have PATH_MAX and the code path is
not hot enough to micro-optimise this allocation.  Revisit if any of
those realities change.

Reviewed-by: DJ Delorie <dj@redhat.com>