[Ada] Implement AI 242

[Arnaud Charlet <charlet at adacore dot com>]

Tested on x86-linux, committed on mainline.

In the Ada RM for Ada 95 (RM B.3.1(49)) there is a statement that a call
to Update with a string parameter is equivalent to the effect of the call
Update(Item, Offset, To_C (Str), Check). But as pointed out in the AI-242
discussion (www.ada-auth.org/cgi-bin/cvsweb.cgi/AIs/AI-00242.TXT?rev=1.6),
this is a clear error with the unintended consequence of truncating the
string after the point of the update. AI-242 specifies the appropriate
correction Update(Item, Offset, To_C (Str, Append_Nul => False), Check).
Since this a correction of a clear error, it seems appropriate to apply
the correction unconditionally, so that it applies in Ada 95 mode as
well. It seems unlikely that any program would be relying on this bug.

2005-01-03  Robert Dewar  <dewar@adacore.com>

	* i-cstrin.adb (Update): Do not append a null in form called with a
	String. This avoids unintended behavior.

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