Fixes and improvements

Fixes:

• Use Z option for volume mount (docker on SELinux systems)
• Fix call to lb-clone-pkg (inside docker)
• Fix use of 'project_deps' before assignment
• Fix decoding of external command outputs
• Fix bug in caching of native env (BINARY_TAG was chopped)
• Fix setup-make.py when some projects are not cloned
• Fix link to vscode documentation
• Fix make with no targets (i.e. defaultProjects setting)

Improvements:

• Initial setup uses distcc only when <24 cores
• Speed up native build-env and make.sh a bit
• Be less strict about having kerberos tickets, closes #24 (closed), #27 (closed)
• Hide misleading message from git remote add
• Document workaround for missing system Python 3
• Document lack of need to make when changing .py

Internal changes:

• Update LbEnv version to 879
• Commit a copy of external tools
• Format external files since lb-format can't exclude them