This commit introduces the OVNI_TRACEDIR environment variable to change
the directory where traces are generated. By default, when the envar is
not defined, the trace is still generated in the ovni directory. The
envar can take a trace directory name, a relative path to the directory,
or its absolute path. In the first case, the directory is created in the
current path $PWD.
Both libovni (rt) and ovnisync read this environment variable.
