Fix warnings 

This pull addresses some more of the most explicit warning messages.
However, there are two cases where some warnings will be left which are out of scope of ettercap and hance left visible.
1) dedundant-delcs: in flex and unistd.h isatty() is redundantly defined.
2) ef_syntax.l and ef_grammar.l inlude both ef_functions.h which declares yylex().  ef_grammar.l is mainly used by bison which does not declare the function yylex() whereas ef_syntax.l is mainly usex by flex which does declare the function yylex(). If ef_function.h declares the function yylex(), warnings redundand-decl is thrown, while deletion thraws implicit-functions-declaration warning. The last one is more severe whereas I accepted to keep the one before splitting ef_functions.h.