Release NUT v2.8.1

Happy Halloween!

Jenkinsfile-dynamatrix: enable linux+mingw cross builds as part of NU…

…T baseline

Replace %jd and %ju with PRI[du]MAX

Quick fixes to build not only for Windows/mingw but also native Linux…

… again ;)

NUT 2.8.0 codebase merged and adjusted with NUT Windows-v2.6.5-8 and …

…beyond, to the point that it can successfully compile

tools/nut-scanner/Makefile.am: link net-snmp statically on (MinGW) WI…

…N32 builds

server/upsd.c: set_exit_flag(): sig is not UNUSED

configure.ac: cut the release of NUT v2.8.0

docs/nut-qa.txt: mention "make check-NIT-devel" helper target

docs/FAQ.txt: add more reasons and symptoms that a running driver dis…

…appears