tzset.3: Filespec omitted incorrect

tzset.3: Filespec omitted incorrect

Paragraph three of the DESCRIPTION section says
that when TZ is set, but empty, then UTC is used.

Later it says if the TZ filespec is omitted then the file
/usr/share/zoneinfo/localtime is used.  This is incorrect,
it will use UTC in that case as well.

Steps to reproduce:

cd /usr/share/zoneinfo/
cp Antarctica/South_Pole localtime
cd
date
Sun Jan 18 10:59:50 EST 2015
TZ=:Hongkong date
Sun Jan 18 23:59:56 HKT 2015
TZ=:/usr/share/zoneinfo/localtime date
Mon Jan 19 05:00:03 NZDT 2015
TZ=: date
Sun Jan 18 16:00:11 UTC 2015
TZ=":" date
Sun Jan 18 16:00:18 UTC 2015
TZ=':' date
Sun Jan 18 16:00:24 UTC 2015
TZ=:/ date
Sun Jan 18 16:00:34  2015
TZ=":/" date
Sun Jan 18 16:00:40  2015
TZ="" date
Sun Jan 18 16:00:45 UTC 2015

Signed-off-by: J William Piggott <elseifthen@gmx.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>