checksrc: introduce 'banfunc' to ban specific functions

checksrc: introduce 'banfunc' to ban specific functions

Use 'banfunc' and 'allowfunc' in .checksrc to specify which functions to
ban or allow to be used. This saves us from having to edit the script
going forward when we want to ban or allow specific functions.

This replaces a set of previous rules and all banned functions are now
checked with the BANNEDFUNC rule.

There is a set of default banned functions, shown by invoking
./checksrc.

Also, -a and -b options are added to specify allowed or banned functions
on the command line.

Closes #15835