This is partly a fix for tickets #222 and #449, although both
of those still need work from RobL.

Reviewed by robl@.

 - A general mechanism for supporting keyval attribute copy and delete callbacks. Instead of setting a language type for a keyval object, a language specific proxy callback function is registered with the keyval. This proxy func gets called when attribute copy/delete is executed on the keyval. The proxy callback function performs the language specific operations and invokes the user registered callback functions.

 - Adding proxy functions to copy/delete attributes for C. C++, fortran

 - Adding MPIR_Keyval_set_proxy() to set the proxy functions for keyvals

 - Adding a new header file, mpi_lang.h, which contains the lang-specific func prototypes

 Contributed by Erez@MS

 Review@thakur,gropp

[svn-r3349] Added the ability to check for handles allocated but not freed; also a number of fixes to plug leaks in handles that this capability exposed

[svn-r3177] Switch to the new macro for the global thread critical section (most of this is an automated change, tested against the MPICH2 test suite)