nrpe_wrapper: Modernize script

The previous version used /bin/bash without actually using bashisms and required check_nrpe to be in /usr/lib/nagios/plugins.

To modernize the script, this commit applies a few changes:

• Bashisms are used where possible (i.e. [[ ]] instead of [ ], $( ) instead of ` `).
• Variables are mostly quoted (just in case...)
• The script changes to the nagios plugin dir to become independent of the nagios plugin installation directory. As a fallback, the hard-coded location for the nagios plugin directory can be used as well (for backwards compatibility).

Closes: #3 (closed)

Closes #3 (closed)