strncpy: implement _FORTIFY_SOURCE=2

Add support for fortify source level 2 to strncpy.
This will enable detection of more areas where strncpy
is used inappropriately. For example, this would have detected
bug 8727221.

Move the fortify_source tests out of string_test.cpp, and
put it into fortify1_test.cpp.

Create a new fortify2_test.cpp file, which copies all
the tests in fortify1_test.cpp, and adds fortify_source level
2 specific tests.

Change-Id: Ica0fba531cc7d0609e4f23b8176739b13f7f7a83
libc: upgrade strrchr to FORTIFY_SOURCE=2

Change-Id: I4c34c2ce22c5092c4446dc1ab55f37604c1c223f