* doc/xml/manual/diagnostics.xml: Document use of errno.
	* doc/html/*: Regenerate.
	* config/locale/generic/c_locale.cc (_Save_errno): New helper.
	(__convert_to_v): Use _Save_errno.
	* include/ext/string_conversions.h (__stoa): Only restore errno when
	it isn't set to non-zero.

From-SVN: r228328