sanitizer: new "strict_string_checks" run-time flag

This patch is related to Issue 346: moar string interceptors: strstr, strcasestr, strcspn, strpbrk
As was suggested in original review http://reviews.llvm.org/D6056 a new "strict_string_checks" run-time flag introduced.
The flag support applied for existing common, asan, msan and tsan interceptors. New asan tests added.

Change by Maria Guseva reviewed in http://reviews.llvm.org/D7123

llvm-svn: 234187