On FreeBSD run nc(1) with -N option (shutdown(2) the network socket
after EOF on the input) if it is available. Otherwise with newer nc(1)
the command hangs.
Use -e option (enable interpretation of backslash escapes) when
echoing usage. This makes output looks as expeted on FreeBSD.
Allow to redefine manager host and port either by specifying via
command line (-h and -p options), or setting environment variables
LEOFS_ADM_HOST and LEOFS_ADM_PORT, or in ~/.leofs-admrc file.