ada: Detect sharing of external file in inconsistent read-write modes

ada: Detect sharing of external file in inconsistent read-write modes

When opening files with "shared=yes", as described in GNAT RM 11.10,
Sharing Files, we now prevent sharing a single file in inconsistent
read-write modes.

gcc/ada/ChangeLog:

	* doc/gnat_rm/the_implementation_of_standard_i_o.rst
	(Shared Files): Add trailing period.
	* libgnat/s-ficobl.ads (AFCB): Reflect new behavior in comment.
	* libgnat/s-fileio.adb (Open): Detect inconsistent sharing,
	just like we do in System.File_IO.Reset.
	* gnat_rm.texi: Regenerate.
	* gnat_ugn.texi: Regenerate.