Issue Description:

llbuild has a number of places where it prints out a command line to the console. When doing so, we should quote the command such that it will execute properly if rerun from the shell, which requires performing shell escaping.

For example:

$ cat t.llbuild 
client:
  name: basic
  version: 0

targets:
  "": ["<dummy>"]

commands:
  C.dummy:
    tool: shell
    args: ["echo", "hello \"world\"!"]
    outputs: ["<dummy>"]

$ llbuild buildsystem build --no-db -f t.llbuild 
echo "hello "world"!"
hello "world"!

The first command printed there echo "hello "world"!" is not escaped properly.