Allow rootless execution without shellscript

[ktock]

This commit enables rootless execution without shellscript buildg.sh.
rootlesskit and slirp4netns are still needed.
Arguments for rootlesskit are configurable via --rootlesskit-args or BUILDG_ROOTLESSKIT_ARGS.
Default arguments for rootlesskit are same as ones of buildg.sh

$ buildg debug /tmp/ctx
WARN[2022-08-30T11:50:06+09:00] using host network as the default            
#1 [internal] load .dockerignore
#1 transferring context: 2B done
#1 DONE 0.0s

#2 [internal] load build definition from Dockerfile
#2 transferring dockerfile: 111B done
#2 DONE 0.1s

#3 [internal] load metadata for ghcr.io/stargz-containers/busybox:1.32.0-org
#3 ...

#4 [auth] stargz-containers/busybox:pull token for ghcr.io
#4 DONE 0.0s

#3 [internal] load metadata for ghcr.io/stargz-containers/busybox:1.32.0-org
#3 DONE 1.1s
INFO[2022-08-30T11:50:08+09:00] CACHED [1/2] FROM ghcr.io/stargz-containers/busybox:1.32.0-org@sha256:bde48e1751173b709090c2539fdf12d6ba64e88ec7a4301591227ce925f3c678 
INFO[2022-08-30T11:50:08+09:00] debug session started. type "help" for command reference. 
Filename: "Dockerfile"
 =>   1| FROM ghcr.io/stargz-containers/busybox:1.32.0-org
      2| RUN echo hello > /hello
(buildg) n

#5 [1/2] FROM ghcr.io/stargz-containers/busybox:1.32.0-org@sha256:bde48e1751173b709090c2539fdf12d6ba64e88ec7a4301591227ce925f3c678
#5 resolve ghcr.io/stargz-containers/busybox:1.32.0-org@sha256:bde48e1751173b709090c2539fdf12d6ba64e88ec7a4301591227ce925f3c678 0.0s done
#5 DONE 0.0s

#6 [2/2] RUN echo hello > /hello
INFO[2022-08-30T11:50:09+09:00] detected 127.0.0.53 nameserver, assuming systemd-resolved, so using resolv.conf: /run/systemd/resolve/resolv.conf 
Filename: "Dockerfile"
      1| FROM ghcr.io/stargz-containers/busybox:1.32.0-org
 =>   2| RUN echo hello > /hello
(buildg) exec echo hello
hello