fs-util: add new CHASE_SAFE flag to chase_symlinks()

fs-util: add new CHASE_SAFE flag to chase_symlinks()

When the flag is specified we won't transition to a privilege-owned
file or directory from an unprivileged-owned one. This is useful when
privileged code wants to load data from a file unprivileged users have
write access to, and validates the ownership, but want's to make sure
that no symlink games are played to read a root-owned system file
belonging to a different context.

(cherry picked from commit f14f1806e329fe92d01f15c22a384702f0cb4ae0)

Change-Id: I86dfede53114a23d708744a9c47abc9eaf05a8d1
Related: #1663143