hdf5_ep

[svn-r15766] Modifications for bug #1247 so that the user can specify the driver
to use when opening the external linked target file.
1. Two new public routines are added to H5Plapl.c as well as
"del/copy/close" callbacks for the property itself.
2. Modify H5L_extern_traverse() to use the fapl set via
H5Pset_elink_fapl() and retrieve via H5Pget_elink_fapl().
3. Add 3 tests to links.c to verify H5Pset/get_elink_fapl().

Also fix the compiler warning for the "if condition" in
H5_build_extpath() of H5system.c.