Turn the static check_bad_bits() function used by both Unix and Windows
systems into a "private" function and extract the "private" parts of
lib/isc/fsaccess.c to lib/isc/fsaccess_common_p.h. Instead of including
lib/isc/fsaccess.c from lib/isc/{unix,win32}/fsaccess.c, make the former
an independent C source file.
Rename lib/isc/fsaccess.c to lib/isc/fsaccess_common.c to prevent build
issues on Windows caused by multiple source files (lib/isc/fsaccess.c,
lib/isc/win32/fsaccess.c) being compiled into the same object file.
These changes improve consistency with the way "private" functions and
macros are treated elsewhere in the source tree.
