fixes DEL in acme resulting in "stringwidth: bad character set for rune 0x0000 ..."

many of *.2400 subfonts in /lib/font/bit/fixed do not have an image
defined for pos 0.  Having it mounted as pos 0 yields NUL's image
(which itself serves as a fall-back image) undefined.  Mounting the
subfonts at pos 1 with the offset compensated effectively eliminates
the problem.