If one uses -L $PATH to point to a full chroot, the startup time
is significant.  In addition, the existing probing algorithm fails
to handle symlink loops.

Instead, probe individual paths on demand.  Cache both positive
and negative results within $PATH, so that any one filename is
probed only once.

Use glib filename functions for clarity.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Tested-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20190519201953.20161-2-richard.henderson@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>