First step in fixing "ex_data" support. Warning: big commit log ...

Currently, this change merely addresses where ex_data indexes are stored
and managed, and thus fixes the thread-safety issues that existed at that
level. "Class" code (eg. RSA, DSA, etc) no longer store their own STACKS
and per-class index counters - all such data is stored inside ex_data.c. So
rather than passing both STACK+counter to index-management ex_data
functions, a 'class_index' is instead passed to indicate the class (eg.
CRYPTO_EX_INDEX_RSA). New classes can be dynamically registered on-the-fly
and this is also thread-safe inside ex_data.c (though whether the caller
manages the return value in a thread-safe way is not addressed).

This does not change the "get/set" functions on individual "ex_data"
structures, and so thread-safety at that level isn't (yet) assured.
Likewise, the method of getting and storing per-class indexes has not
changed, so locking may still be required at the "caller" end but is
nonetheless thread-safe inside "ex_data"'s internal implementation.
Typically this occurs when code implements a new method of some kind and
stores its own per-class index in a global variable without locking the
setting and usage of that variable. If the code in question is likely to be
used in multiple threads, locking the setting and use of that index is
still up to the code in question. Possible fixes to this are being
sketched, but definitely require more major changes to the API itself than
this change undertakes.

The underlying implementation in ex_data.c has also been modularised so
that alternative "ex_data" implementations (that control all access to
state) can be plugged in. Eg. a loaded module can have its implementation
set to that of the application loaded it - the result being that
thread-safety and consistency of "ex_data" classes and indexes can be
maintained in the same place rather than the loaded module using its own
copy of ex_data support code and state.

Due to the centralisation of "state" with this change, cleanup of all
"ex_data" state can now be performed properly. Previously all allocation of
ex_data state was guaranteed to leak - and MemCheck_off() had been used to
avoid it flagging up the memory debugging. A new function has been added to
perfrom all this cleanup, CRYPTO_cleanup_all_ex_data(). The "openssl"
command(s) have been changed to use this cleanup, as have the relevant test
programs. External application code may want to do so too - failure to
cleanup will not induce more memory leaking than was the case before, but
the memory debugging is not tricked into hiding it any more so it may
"appear" where it previously did not.