Docs: Explain `nrpe_arguments` in NRPE section

People still use it, and it's the best place there, instead of
the general documentation which should not get messed up with
NRPE at all.

refs #5474
Use relative paths for the OpenSSL DLLs
Update minimum GCC version
Replace BOOST_FOREACH with range-based for loops
Use C++11 atomics for the ThinMutex and Object classes