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.