tail: avoid outputting repeated data with remote files

tail: avoid outputting repeated data with remote files

* src/tail.c (tail_forever): Only read up to st_size on network
file systems to avoid the issue with a stale attribute cache
returning a smaller st_size than we have already read().
The was seen with glusterfs at least and caused the complete
file to be repeatedly output due to assuming the file was
truncated in this case.
* NEWS: Mention the fix.