Make 'dub remove' remove all packages by default in non-interactive mode ...

Given how often it is used in the test-suite, it should be self-obvious
that it will be what most users want.

testsuite: Replace a non-portable usage of rm (!) with dub clean ...

Who would have known that the following would pass on Linux but not Mac OSX:
---
touch file
rm -f file/nonono
---
Well surely not me. And indeed "b" is a file, not a directory,
so this rm didn't make sense. Just use dub clean. Like the rest of us.

fix failures (due to bash error flags)

unify flags and error handling ...

- also add trap ERR handler to report exact error location

Add test for #990.