timedated: gather timezone from /etc/localtime sym target

/etc/localtime -> /usr/share/zoneinfo/...

or

/etc/localtime -> ../usr/share/zoneinfo/...

(note, ../usr is not the same if /etc is a symlink, as this isn't
using canonicalize_file_name())

keep other method for now, consider dropping later.

Supporting relative links here are problematic as timezones in
/usr/share/zoneinfo are often themselves symlinks (and symlinks to
symlinks), so this implamentation only supports absolute symlinks
"/usr/share/zoneinfo/" and relative symlinks starting with
"../usr/share/zoneinfo/"

>From TODO (kay sievers):
* kill /etc/timezone handling entirely? What does it provide?
  - /etc/localtime carries the same information already:
      $ ls -l /etc/localtime; cat /etc/timezone
      lrwxrwxrwx 1 root root 33 Jul 27 09:55 /etc/localtime -> /usr/share/zoneinfo/Europe/Berlin
      Europe/Berlin
  - systemd enforces /usr to be available at bootup, so we can
    enforce the use of the symlink
man: remove timezone(5) and add localtime(5)
timedate: assorted improvements

- Make writing/reading of /etc/timezone dependendent of HAVE_SYSV_COMPAT

- Introduce symlink_atomic() after all, and use it

- Use relative symlink for /etc/localtime
timedated: make /etc/timezone a Debian-only thing
man: mention journalctl in the systemd man page
man: update localtime(5) a bit
update TODO
TODO: isolate items to fix before F18
update TODO
logind: redefine idleness to start at last activity

Before, after the timeout, a session would be timestamped as idle
since 'last activity' + 'idle timeout'. Now, it is timestamped as idle
since 'last activity'.

Before, after all sessions were idle, the seat would be marked with as
idle with the timestamp of the oldest idle session. Now it is
marked with the timestamp of the youngest idle session.

Both changes seem to me to be closer to natural understanding of
idleness: the time since last activity counts.
install: treat non-existent directory as empty

When looking for symlinks, it doesn't make sense to error-out if
the directory is missing. The user might delete an empty directory.

This check caused test-unit-file to fail when run before installation.
install: use automatic cleanup
nspawn: mount tmpfs on /dev/shm

Most things seem to function fine without /dev/shm, but it is expected
to be there (quoting linux/Documentation/filesystems/tmpfs.txt:
glibc 2.2 and above expects tmpfs to be mounted at /dev/shm for POSIX
shared memory (shm_open, shm_unlink)).

Since /tmp/ is already mounted as tmpfs, it would be enough to mkdir
/tmp/shm and chmod it. Mounting it separately has the advantage that
it can be easily remounted to change the quota.
nspawn: use automatic cleanup

This one actually clears up a (totally harmless) memleak.
nspawn: _cleanup_free_ more
nspawn: use automatic cleanup for umask
nspawn: fix memleak introduced with automatic cleanup

6b2d0e8 introduced a memleak instead of fixing one.
Fix both.
socket: prevent signed integer overflow

src/core/socket.c:588:25: error: overflow in implicit constant conversion
src/core/socket.c:589:17: error: overflow in implicit constant conversion
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.
build-sys: __secure_getenv lost dunder in libc 2.17