Append success/failure commands of latexmk instead of overwriting.

[Nicholas42]

Append success and failure callbacks to success_cmd and failure_cmd instead of overwriting them. This allows to use them in a latexmkrc-file, where right now they are overwritten and hence not executed. This fixes #1237.

I introduce a function wrap_append_cmd_option that only differs from wrap_option by

1. using .= instead of = which is appending in perl
2. adding a ; before the appended value. This separates shell commands. It is no problem if the variable was empty beforehand.

I am unsure if this works correctly on windows since I never did scripting there and do not know if ; has the same effect. One could easily use the old variant for windows if this does not work.

The change only influences success_cmd and failure_cmd since I did not change wrap_option here. In my opinion overwriting is the right thing to do at the other places it is used.

[lervag]

Thanks! I don't see any reason not to merge this. And I appreciate your contribution!

[lervag]

See #2078: I've had to revert this, because it breaks things when the ; becomes the leading part of a command.