A general spring-cleaning (in autumn) to fix up signed/unsigned warnings.

I have tried to convert 'len' type variable declarations to unsigned as a
means to address these warnings when appropriate, but when in doubt I have
used casts in the comparisons instead. The better solution (that would get
us all lynched by API users) would be to go through and convert all the
function prototypes and structure definitions to use unsigned variables
except when signed is necessary. The proliferation of (signed) "int" for
strictly non-negative uses is unfortunate.