SystemTools: Clarify/simplify behavior of FileExists

The FileExists function always ensures that a file is readable on
POSIX systems by using access() with R_OK.  This was due to bug #809.
Publicly state in the header file that this is being done.

Also simplify the implementation so that redundant code handling the
isFile parameter is eliminated.

Change-Id: Ie07bec7aa5dd2bca07bb3244c01c0a29587aa437