uuid plugin: Improve parsing of the dmidecode output.

uuid plugin: Improve parsing of the dmidecode output.

The entire business of dividing by sizeof(char) only made the code
unreadable, because by specification sizeof(char) evaluates to 1. The
defines were expanded and the parsing is now less sensitive to
formatting changes in the dmidecode output. Module global variables have
been moved to the top.

Fixes Github issue #267 in the process.