Fix most warnings in libdap2

[ZedThree]

I think most of the remaining warnings in libdap2 are suspicious. For example, there are some (duplicated, but that's a different matter) functions for finding the index of an element in an NClist that return -1 if the element isn't found. This is never checked, and is used as a size_t. There are various options for fixing this, but I've left them as-is, or changed types so that the warning is a much more explicit:

warning: unsigned conversion from ‘int’ to ‘size_t’ {aka ‘long unsigned int’} changes value from ‘-1’ to ‘18446744073709551615’

[edwardhartnett]

Thanks for all this warnings work!

Every time we release code with warnings, an angel kicks a kitten right in the face.

[ZedThree]

In my branch with everything merged, I'm down to 370 warnings from 2405 in November.

[ZedThree]

There are 11 warnings remaining in libdap2 and they're all suspicious, for example:

static size_t
findin(CDFnode* parent, CDFnode* child)
{
    NClist* subnodes = parent->subnodes;
    for(size_t i=0;i<nclistlength(subnodes);i++) {
	if(nclistget(subnodes,i) == child)
	    return i;
    }
    return -1;  // Conversion warning
}

There's also no checking at the call site, so I guess this is expected to always find child? In which case, maybe it's better to hard error here?

The other warnings are similar, indicating missing checks.