Refactor: daemons: Use pcmk__request_t for CRM_OP_REGISTER.

This commit is typical of how the commit for each command is going to
work:

• Expose a function in execd_commands.c to the rest of the exec daemon and give it a name that is consistent with "public" functions.

• Have it return a standard pacemaker return code.

• Remove the old handler block from process_lrmd_message (this is the older function, not the newer confusingly named function).

• Add a new handler function and add that function to the handler table.