common: use /dev/null as stdin when calling subprocess.Popen()

common: use /dev/null as stdin when calling subprocess.Popen()

We always want to run all utilities non-interactively. By default
subprocess.Popen() inherits stdin descriptor from parent process, i.e.
when fdroid is run from an interactive shell, subprocesses may expect
input from it.

Reading from /dev/null immediately returns EOF, failing any user prompt
and preventing us from hang.