nut: add dev headers, nut-web-cgi and nut-avahi-service, bugfix

nut: add dev headers, nut-web-cgi and nut-avahi-service, bugfix

Headers are required by collectd as mentioned in #1801. This is the nut portion based on dwmw2@d636841cd1d1dfab04cb509520c082738ddfb2ea
Happy to merge with 15.05 as per #1803.
Signed-off-by: David Woodhouse <redacted>
[martin.p.rowe@gmail.com: condensed InstallDev lines]

Added package nut-web-cgi based on demorfi@7e340f47944ff4a9c49d3b1dc6c1d9d965cd344c. Closes #1614.
Pending signoff by @demorfi
[martin.p.rowe@gmail.com: simplified implementation, no changes to Config.in]

Added package nut-avahi-service similar to other implementations in #618
Bugfix to remove redundant NUT_DRIVER_SERIAL config
Bugfix to make UPSLOG config actually install (requires PKG_RELEASE bump)
Some code tidy-ups
Signed-off-by: Martin Rowe <redacted>