appropriate code if any of them is defined.

This change adds some basic control commands to the existing ENGINEs
(except the software 'openssl' engine). All these engines currently load
shared-libraries for hardware APIs, so they've all been given "SO_PATH"
commands that will configure the chosen ENGINE to load its shared library
from the given path. Eg. by calling;
    ENGINE_ctrl_cmd_string(e, "SO_PATH", <path>, 0).

The nCipher 'chil' ENGINE has also had "FORK_CHECK" and "THREAD_LOCKING"
commands added so these settings could be handled via application-level
configuration rather than in application source code.

Changes to "openssl engine" to test and examine these control commands will
be made shortly. It will also provide the necessary tips to application
programs wanting to support these dynamic control commands.

The existing ENGINEs (including the default 'openssl' software engine) were
static, declared inside the source file for each engine implementation. The
reason this was not going boom was that all the ENGINEs had reference
counts that never hit zero (once linked into the internal list, each would
always have at least 1 lasting structural reference).

To fix this so it will stay standing when an "unload" function is added to
match ENGINE_load_builtin_engines(), the "constructor" functions for each
ENGINE implementation have been changed to dynamically allocate and
construct their own ENGINEs using API functions. The other benefit of this
is that no ENGINE implementation has to include the internal "engine_int.h"
header file any more.

ENGINE handler functions should take the ENGINE structure as a parameter -
this is because ENGINE structures can be copied, and like other
structure/method setups in OpenSSL, it should be possible for init(),
finish(), ctrl(), etc to adjust state inside the ENGINE structures rather
than globally. This commit includes the dependant changes in the ENGINE
implementations.

sure they are available in opensslconf.h, by giving them names starting
with "OPENSSL_" to avoid conflicts with other packages and by making
sure e_os2.h will cover all platform-specific cases together with
opensslconf.h.

I've checked fairly well that nothing breaks with this (apart from
external software that will adapt if they have used something like
NO_KRB5), but I can't guarantee it completely, so a review of this
change would be a good thing.

engine structure, and some ANSI C compilers will complain otherwise.

few small constifying changes, and why not throw in a couple of extras
while I'm at it?

At the same time, add VMS support for Rijndael.