[Ada] Fix possible infinite recursion in directory iterator

[Pierre-Marie de Rodat]

When the directory iterator Find is called we need to ensure that
symbolic links are skipped to avoid possible circularities or exploring
unrelated directories.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* libgnat/g-diopit.adb (Find): Fix possible infinite recursion
	in Find iterator.
	* libgnat/g-diopit.ads (Find): Update comments accordingly.
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