lnstat: fix header displaying mechanism

lnstat: fix header displaying mechanism

The algorithm depends on the loop counter ('i') to increment by one in
each iteration. Though if running endlessly (count==0), the counter was
not incremented at all.

Also change formatting of the header printing conditional a bit so it's
hopefully easier to read.

Fixes: e7e2913 ("lnstat: run indefinitely by default")
Signed-off-by: Phil Sutter <phil@nwl.cc>