Unify behavior of mode bits for files/directories.

Previously the prescribed behavior by chirp_reli.h mandated that only the user
executable bit could be set. However, the chirp_fs_local.c implementation
allowed setting group/other bits and the executable bit can only be changed if
the file is not a directory.

This commit explicitly documents the user may set group/other bits and the user
executable bit (for non-directories). All backend file systems implement this
except HDFS, which has no executable bit.