Implement the -x option for makechrootpkg which allows to get an
interactive shell in the chroot after building the package. Useful to
ease the debugging of a package build.

Depending on the argument, the interactive shell is either always
spawned or only when an error occurred during build.

This option is also forwarded from `pkgctl build` via the `--inspect`
flag.

Component: pkgctl build
Component: makechrootpkg
Signed-off-by: Levente Polyak <anthraxx@archlinux.org>