Refactor macro kludge in command.cpp fxn `executeret`

[no-lex]

https://github.com/project-imprimis/imprimis/blob/2f7b46f4fde9e42151d0484f38e71311cffc5149/src/engine/command.cpp#L2881-L2893

This relies on the macro assignment #define callargs being identical to the int callargs assigned in the nominal usage of the macro in https://github.com/project-imprimis/imprimis/blob/2f7b46f4fde9e42151d0484f38e71311cffc5149/src/engine/command.cpp#L2711-L2754

This is bad because it

• relies on the macro imitating a normal variable
• has a macro with the same name as a normal variable
• reinterprets an argument of a macro with another macro to be an unrelated variable
• reinvents what a function is in the C++ preprocessor
• is entirely undocumented

This should be replaced with an inline function doing the same thing.