tmpfiles: use write(2) for the 'w' action

This resolves problems with filesystems which do not implement the
aio_write file operation. In this case, the kernel will fall back using
a loop writing technique for each pointer in a received iovec. The
result is strange errors in dmesg such as:

[   31.855871] elevator: type  not found
[   31.856262] elevator: switch to
[   31.856262]  failed

It does not make sense to implement a synchronous aio_write method for
sysfs as this isn't a real filesystem where a reasonable use case for
using writev exists, nor is there an expectation that tmpfiles will be
used to write more data than can be reasonably written in a single write
syscall.

In addition, some sysfs attrs are currently buggy and will NOT reject
the second write with the newline, causing the sysfs value to be zeroed
out. This of course should be fixed in the kernel regardless of any
wrongdoing in userspace, but this simple change makes us immune to such
a bug.

This change means that we do not write a trailing newline by default, as
the expected use case of 'w' is for sysfs and procfs. In exchange, honor
C-style backslash escapes so that if the newline is really needed, the
user can add it.