... and fail the VCL unless successful.

Providing the PRIVs to vmods is a core function, so error handling
should happen outside vmods.

Besides being safe, this initialization can be more efficient than
previous code for PRIVs used frequently within the same subroutine.

An alternative approach would be to initialize all privs once per
task / top request, but unless all privs are actually used in a VCL,
this approach could impose significant overhead, both in terms of time
and memory. By initializing privs once per sub, we impose overhead for
privs which are referenced but not actually used in a subroutine, but
not for all of the vcl.

Fixes #2708