chroot/repos-local.conf: do not overwrite existing xbps repos.
libexec/xbps-src-dostrip.sh: remove "pkg-" prefix from generated debug pkgs.
libexec/xbps-src-doinstall.sh: generate pkg DESTDIR correctly in cross builds.
shutils/chroot.sh: copy xbps host keys to target cross base only in cross builds.
Misc tweaks.