Add tests for constants and do other macro niceties. - Adds tests for all the constants. Pretty sweet. - Moves documentation for cfg-pam-impl macro to `libpam-sys`. - Renames `Illumos` to `Sun`. - other stuff

FROM docker.io/debian:stable

RUN apt-get -y update \
    && apt-get -y upgrade \
    && apt-get install -y cargo mercurial \
    && apt-get clean