[sanitizer] Intercept fgetpwent / fgetgrent.

These interceptors require deep unpoisoning of return values.
While at it, we do the same for all other pw/gr interceptors to
reduce dependency on libc implementation details.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@205004 91177308-0d34-0410-b5e6-96231b3b80d8