Do not override $MANWIDTH if it is set.

From Linux man(1) page:

MANWIDTH
   If $MANWIDTH is set, its value is used as the  line  length  for
   which  manual pages should be formatted.  If it is not set, man‐
   ual pages will be formatted with a line  length  appropriate  to
   the  current terminal (using an ioctl(2) if available, the value
   ...

I have set $MANWIDTH to '80' so it is easier to read man pages
in terminals, especially in maximized terminal windows. So this
fix to make manpageviee behaviour consistent with terminals.
Merge pull request #1 from valinskas/master

Do not override $MANWIDTH if it is set.