also support adduser

[jonaz]

Currently installing only supports useradd:

bash-4.4# ./stampzilla install server
PANI[2019-02-18T15:12:47Z] Check user 'stampzilla'                       error="exec: \"useradd\": executable file not found in $PATH" exists=false output=

Could also support adduser

bash-4.4# adduser --help
BusyBox v1.28.4 (2018-07-17 15:21:40 UTC) multi-call binary.

Usage: adduser [OPTIONS] USER [GROUP]

Create new user, or add USER to GROUP

	-h DIR		Home directory
	-g GECOS	GECOS field
	-s SHELL	Login shell
	-G GRP		Group
	-S		Create a system user
	-D		Don't assign a password
	-H		Don't create home directory
	-u UID		User id
	-k SKEL		Skeleton directory (/etc/skel)