[Ada] Refactor repeated complains about missing -gnat2005 switch

[Pierre-Marie de Rodat]

Refactor repeated code in the parser for complaining about a missing
-gnat2005 switch. Fix singular-vs-plural phrasing.

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

gcc/ada/

	* errout.ads (Error_Msg_Ada_2005_Extension): New routine (spec).
	* errout.adb (Error_Msg_Ada_2005_Extension): New routine (body).
	* par-ch10.adb: Reuse new routine; correct casing for "LIMITED
	WITH".
	* par-ch11.adb: Likewise.
	* par-ch12.adb: Likewise.
	* par-ch3.adb: Likewise.
	* par-ch4.adb: Likewise; replace "box" with "<>".
	* par-ch6.adb: Likewise.
	* par-ch9.adb: Likewise; correct casing for "THEN ABORT".
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