collectd.c: Check for errors in sigaction().

collectd.c: Check for errors in sigaction().

Terminate collectd if sigaction() fails which should not happen anyway.

While I was at it, I renamed the signal handler functions and the sigaction
structs to follow the coding style used everywhere else in collectd.

Signed-off-by: Sebastian Harl <sh@tokkee.org>
Signed-off-by: Florian Forster <octo@huhu.verplant.org>