Support special target "~" on cmdline, to request parallel execution

[lefou]

Currently, all targets given on the sbuild commandline will result in serialized execution of these targets. Thus, the commandline sbuild a b is roughly the same as a ~~ b when written as dependencies in a buildscript. This is IMHO a good default. But of course, an commandline analogy to the a ~ b dependency is currently missing.

Writing sbuild a "~" b would be a lightweight and clear way to express that the targets a and b should/could be executed concurrently.

Warning: On most shells, the "~" (tilde) needs to be quoted, to avoid shell expansion to the home directory of the current user, though.