analyzer: implement kf_strncpy [PR105899]

analyzer: implement kf_strncpy [PR105899]

gcc/analyzer/ChangeLog:
	PR analyzer/105899
	* kf.cc (class kf_strncpy): New.
	(kf_strncpy::impl_call_post): New.
	(register_known_functions): Register it.
	* region-model.cc (region_model::read_bytes): Handle unknown
	number of bytes.

gcc/testsuite/ChangeLog:
	PR analyzer/105899
	* c-c++-common/analyzer/null-terminated-strings-2.c: New test.
	* c-c++-common/analyzer/overlapping-buffers.c: Update dg-bogus
	directives to avoid clashing with note from <string.h> that might
	happen to have the same line number.  Add strpncpy test coverage.
	* c-c++-common/analyzer/strncpy-1.c: New test.
	* gcc.dg/analyzer/null-terminated-strings-1.c
	(test_filled_nonzero): New.
	(void test_filled_zero): New.
	(test_filled_symbolic): New.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>