PR c/81117 - Improve buffer overflow checking in strncpy - part 2

PR c/81117 - Improve buffer overflow checking in strncpy - part 2

gcc/ChangeLog:

	PR c/81117
	* doc/extend.texi (attribute nonstring): Document new attribute.

gcc/c-family/ChangeLog:

	PR c/81117
	* c-attribs.c (c_common_attribute_table): Add nonstring entry.
	(handle_nonstring_attribute): New function.

gcc/testsuite/ChangeLog:

	PR c/81117
	* c-c++-common/attr-nonstring-1.c: New test.

From-SVN: r251100