Clean up the remaining sign-conversion issues

This also turns on Wsign-conversion and Wfloat-conversion going
forward, as I think we can and should keep clean on those in the
future.

The broader Wconversion would cover both of these as well as
implicit bit-width downconversions, but currently the code isn't
there yet.  I suspect that becoming Wconversion -clean would
involve a rather large set of changes of the nature of
s/unsigned/size_t/ all over the codebase, all which need careful
examination rather than blind regex-application of the principle.

There are also potential positive and negative performance
fallouts from size_t-ification: the larger widths lead to more
natural/short code sequences in many cases, but it could also
increase data sizes such that there's a negative effect on fitting
things into dcache lines.
Bugfix for extmon timeout/interval >255s

The general monitoring core in gdnsd/libgdnsd/mon.c allows for a
timeout up to 300s and an interval up to 3600s.  However, the
extmon code was truncating these to a single byte for transmission
to the helper over a pipe, which results in the helper using the
raw values modulo 256, which would result in substantially reduced
timeouts and intervals for values >255.

This bug became more-obvious as a result of looking at
type-conversion issues in the network pipe traffic code that
communicates these values.
Fix ac-2.63 compat (Issue #85)