Unprivileged build user

Greg requested that bob use PKG_USERS to create an unprivileged builder 
user by default, which seems entirely reasonable.

I'd like to go one step further and settle on a standard unprivileged 
build user that bob, pbulk, and potentially others in the future can 
all use.  pbulk has shipped this:

   # Account used for user-destdir builds. This account should have
   # no special permissions.
   #
   unprivileged_user=pbulk

in its default pbulk.conf for many years, without creating PKG_USERS, 
and so I expect others may be like me and currently re-use the pbulk 
user or similar that they've previously created for pbulk, for bob.

What I propose is that we add a standard:

   PKGBUILD_USER?=	pkgbuild
   PKGBUILD_GROUP?=	pkgbuild

to mk/defaults/mk.conf, and then update both pbulk and bob to use this 
for their default PKG_USERS/PKG_GROUPS.

I would then add a small bit of SUBST in bob to correctly populate this 
in the generated config.lua so that unprivileged builds work out of the 
box for new users.  As an existing user I'd be happy to modify my 
existing host setups to match this.

Any particular grievances about this before I go ahead?

-- 
Jonathan Perkin                    pkgsrc.smartos.org
Open Source Complete Cloud   www.tritondatacenter.com