Remove unchecked casts to mi_interp

Remove unchecked casts to mi_interp

Simon noticed a crash that could be caused via new Python
gdb.execute_mi function.  Looking into this, I found a few unchecked
casts to mi_interp, like:

-  struct mi_interp *mi = (struct mi_interp *) command_interp ();

This patch replaces all such casts with safer variants.

For -gdb-exit and mi_load_progress, I chose to have the functions
simply not generate any output.  It didn't seem useful to do so.

Some casts I eliminated by adding a parameter to a function.  Then, in
mi_execute_command, I changed the code to use
gdb::checked_static_cast.  This is appropriate because this particular
overload can only be called by the MI interpreter.

There does not seem to be a very good way to test -gdb-exit.

Regression tested on x86-64 Fedora 36.