fix dangling symlink when using SCM [BZ# 758781]
    remove setting TMPDIR in chroot environment [BZ# 769728]
    add code to allow global proxy in chroot [BZ# 766199]
    explicitly set unprivileged umask in --shell [BZ# 747119]
    add bind-mount config to create sourcedirs [BZ# 706174]
    move mount management into classes
    update environment management code