Invert the relationship of `lib/srv` and `lib/bpf`,
`lib/restrictedsession` such that `lib/bpf` is only imported in
`lib/srv/regular`. Since not everything is built with the `bpf`
tag it's important to reduce the surface area of `lib/bpf` such
that it isn't inadvertantly imported. For instance it was entirely
possible to import a package in `tsh` that transitively
depends on `lib/bpf` - which breaks the build since `tsh` is not
compiled with the `bpf` tag.
By refactoring the `srv.Server` interface not to use the `bpf.BPF`
and `restrictedsession.Manager` interfaces directly anything that
imports `lib/srv` now won't require that `-tags=bpf` is set in
order to compile.
Nic Klaassen