After cleanup introduced with previous commit, there is a need for
syntax-check rule taking care of return(). Regexp used in 'prohibit'
parameter is taken from the cleanup commit and modified so it fits
'grep -E' format. Semicolon at the end is needed, otherwise the regexp
could match return with cast.

Exception is created for python source files because we don't have any
documentation restricting the use of return that matches this case.