os/user: fix user lookups on dragonfly
os/user: fix user lookups on dragonfly

Like FreeBSD, DragonFly does not provide a sysconf value for
_SC_GETPW_R_SIZE_MAX.

R=golang-dev, iant
CC=golang-dev
https://codereview.appspot.com/14469043

Committer: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme