adduser: Normalize supplemental groups

Rather than insist on a space-separated list, normalize the input so
that any number of spaces, commas, colons, or semicolons are accepted.

Reviewed by: bcr
Differential Revision: https://reviews.freebsd.org/D57774

(cherry picked from commit 65fba082f44cb9451d2b12bcd99c3287d0a47acc)