hdf5_ep

[svn-r17002] Description:
    Rename H5O_protect/H5O_unprotect to be H5O_pin/H5O_unpin, since that's what
that are actually doing.

    Add counter of the number of times the object header is pinned, to allow
H5O_pin/H5O_unpin to be called reentrantly.

Tested on:
    FreeBSD/32 6.3 (duty) in debug mode
    FreeBSD/64 6.3 (liberty) w/C++ & FORTRAN, in debug mode
    Linux/32 2.6 (jam) w/PGI compilers, w/C++ & FORTRAN, w/threadsafe,
        in debug mode
    Linux/64-amd64 2.6 (smirom) w/Intel compilers w/default API=1.6.x,
        w/C++ & FORTRAN, in production mode
    Solaris/32 2.10 (linew) w/deprecated symbols disabled, w/C++ & FORTRAN,
        w/szip filter, in production mode
    Linux/64-ia64 2.6 (cobalt) w/Intel compilers, w/C++ & FORTRAN,
        in production mode
    Linux/64-ia64 2.4 (tg-login3) w/parallel, w/FORTRAN, in debug mode
    Linux/64-amd64 2.6 (abe) w/parallel, w/FORTRAN, in production mode
    Mac OS X/32 10.5.7 (amazon) in debug mode
    Mac OS X/32 10.5.7 (amazon) w/C++ & FORTRAN, w/threadsafe,
        in production mode