nagios-nrpe; various fixes for systemd

1. The PID file cannot be created in the default installation. The .cfg file
says /var/run/nrpe.pid, but normal users ("nrpe" in this case) may not write to
/var/run or /run (anymore). The journal contains the following:

Okt 04 11:28:44 sweet-chili nrpe[2014]: Starting up daemon
Okt 04 11:28:44 sweet-chili nrpe[2014]: Cannot write to pidfile
    '/var/run/nrpe.pid' - check your privileges.

Fix consists of three steps: a) Include a tmpfile spec for systemd named
/usr/lib/tmpfiles.d/nrpe with the following content:
    d /run/nrpe 0755 nrpe nrpe -

b) Fix nrpe.cfg to have its pid_file point to /run/nrpe/nrpe.pid
c) call "systemd-tmpfiles --create" in your postinstall script

2. The conf.d directory is not found, most likely because its location is
relative and systemd starts nrpe after chdir()ing to /. Fix is easy, adjust the
include_dir entry to contain the full path /etc/nrpe/conf.d/

journal entries for this error message:

Okt 04 11:37:38 sweet-chili nrpe[3289]: Could not open config directory
    'conf.d' for reading.
Okt 04 11:37:38 sweet-chili nrpe[3289]: Continuing with errors...
nagios-nrpe; fix quoting and minor fixes for previous commit
nagios-nrpe; fix md5sums