tailf: count last lines correctly at initial print out

tailf: count last lines correctly at initial print out

When last lines happen to be greater than string buffer size for fgets()
the number of printed lines resulted to too few.  To avoid miscounts due
insufficient buffer size use mmap() to map the whole file and rewind
until requested number of new lines is found.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>