ENGINE.

 * Extra verbosity can be added with more "v"'s, eg. '-vvv' gives
   information about input flags and descriptions for each control command
   in each ENGINE. Check the output of "openssl engine -vvv" for example.

 * '-pre <cmd>' and '-post <cmd>' can be used to invoke control commands on
   the specified ENGINE (or on all of them if no engine id is specified,
   although that usually gets pretty ugly). '-post' commands are only
   attempted if '-t' is specified and the engine successfully initialises.
   '-pre' commands are always attempted whether or not '-t' causes an
   initialisation to be tried afterwards. Multiple '-pre' and/or '-post'
   commands can be specified and they will be called in the order they
   occur on the command line.

Parameterised commands (the normal case, there are currently no
unparameterised ones) are split into command and argument via a separating
colon. Eg. "openssl engine -pre SO_PATH:/lib/libdriver.so <id>" results in
the call;
    ENGINE_ctrl_cmd_string(e, "SO_PATH", "/lib/libdriver.so", 0);

Application code should similarly allow arbitrary name-value string pairs
to be passed into ENGINEs in a manner matching that in apps/engine.c,
either using the same colon-separated format, or entered as two distinct
strings. Eg. as stored in a registry. The last parameter of
ENGINE_ctrl_cmd_string can be changed from 0 to 1 if the command should
only be attempted if it's supported by the specified ENGINE (eg. for
commands like "FORK_CHECK:1" that may or may not apply to the run-time
ENGINE).