Fix string truncation warnings related to PATH_MAX (#1244)

Fix string truncation warnings related to PATH_MAX (#1244)

There are a number of places where rrdtool combines multiple PATH_MAX
sized strings into one.

PATH_MAX is a constant that tends to work in practice a lot of the time
but may not reflect the real capabilities of the system in real time.

In place of on-stack buffers of PATH_MAX size allocate memory
dynamically. Initialize the pointers to NULL so they can be all freed
unconditionally on exit.

Fixes: #1223
Signed-off-by: Michal Suchanek <msuchanek@suse.de>