MINOR: cfgparse: add two new functions to check arguments count

MINOR: cfgparse: add two new functions to check arguments count

We already had alertif_too_many_args{,_idx}(), but these ones are
specifically designed for use in cfgparse. Outside of it we're
trying to avoid calling Alert() all the time so we need an
equivalent using a pointer to an error message.

These new functions called too_many_args{,_idx)() do exactly this.
They don't take the file name nor the line number which they have
no use for but instead they take an optional pointer to an error
message and the pointer to the error code is optional as well.
With (NULL, NULL) they'll simply check the validity and return a
verdict. They are quite convenient for use in isolated keyword
parsers.

These two new functions as well as the previous ones have all been
exported.