goyacc: fix units.y build breakage

goyacc: fix units.y build breakage
This breakage is mainly due to API changes in pkg.
(e.g., package utf8 moved to unicode/utf8;
       remove of strconv.Atof64;
       change character type from int to rune.)
Also correct the usage comment.

This fixes issue 2646.
PS: I don't change the goyacc.go, because I think token type
    should not be force to rune.

R=golang-dev, adg, rogpeppe, r, r
CC=golang-dev
https://golang.org/cl/5502093