nspawn-mount: rework get_controllers() a bit

nspawn-mount: rework get_controllers() a bit

Let's rename get_controllers() → get_process_controllers(), in order to
underline the difference to cg_kernel_controllers(). After all, one
returns the controllers available to the process, the other the
controllers enabled in the kernel at all).

Let's also update the code to use read_line() and set_put_strdup() to
shorten the code a bit, and make it more robust.