Add a B_EXECUTE_PROPERTY to execute a given command.
Also set fAttached to false in _DetachShell() for symmetry.
Done by Andrea Anzani plus some style / refactoring by myself
Change-Id: Ifa26b60b1c1d3598a863adc4fb701155f9edf588
Reviewed-on: https://review.haiku-os.org/c/haiku/+/9154
Tested-by: Commit checker robot <[email protected]>
Reviewed-by: waddlesplash <[email protected]>