pw: Don't return a pointer to an on-stack buffer from grp_set_passwd.

Make 'line' static to move it to .bss instead as that pattern is used
elsewhere in pw(8) (e.g. the static buffer in pw_pwcrypt).

Reported by: GCC -Wdangling-pointer
Reviewed by: imp, emaste
Differential Revision: https://reviews.freebsd.org/D37534

(cherry picked from commit 1055bedbed42dc176468b45b4f167a752ed8868f)