cgroup: Handle multiline properties in restore_cgroup_prop()

We need to write some multiline properties line-by-line,
instead of writing all buffer at once - because kernel doesn't
support parsing them in one write().

Previously, it was done for devices.allow list, but I need it
also for netprio.ifpriomap. To make easier reusing this code,
add split_lines parameter to restore_cgroup_prop().

Dropping next_device_entry() and multiply calls to
restore_cgroup_prop() also improves performance: previously
for each property's line, cgroup file was re-open() and close()
again, while new code only calls write() for each line, reducing
number of syscalls done for each line.

This also makes it suitable for future:
if patch(s) for parsing all lines of property with one write()
would be accepted, then we can easily change call to something:
restore_cgroup_prop(&dev_allow, paux, off, !kdat.cgroup_multiline_write);

We can even use split_lines = true by default for all cases,
as strchrnul() will only find zero-terminator as does strlen().
But I don't want to do it - if there are cgroup files which
supportes multi-line write(), it'll add syscalls.

Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>