linux-user: Fix syslog() syscall support

There are currently several problems related to syslog() support.

For example, if the second argument "bufp" of target syslog() syscall
is NULL, the current implementation always returns error code EFAULT.
However, NULL is a perfectly valid value for the second argument for
many use cases of this syscall. This is, for example, visible from
this excerpt of man page for syslog(2):

> EINVAL Bad arguments (e.g., bad type; or for type 2, 3, or 4, buf is
>        NULL, or len is less than zero; or for type 8, the level is
>        outside the range 1 to 8).

Moreover, the argument "bufp" is ignored for all cases of values of the
first argument, except 2, 3 and 4. This means that for such cases
(the first argument is not 2, 3 or 4), there is no need to pass "buf"
between host and target, and it can be set to NULL while calling host's
syslog(), without loss of emulation accuracy.

Note also that if "bufp" is NULL and the first argument is 2, 3 or 4, the
correct returned error code is EINVAL, not EFAULT.

All these details are reflected in this patch.

"#ifdef TARGET_NR_syslog" is also proprerly inserted when needed.

Support for Qemu's "-strace" switch for syslog() syscall is included too.

LTP tests syslog11 and syslog12 pass with this patch (while fail without
it), on any platform.

Changes to original patch by Riku Voipio:

 fixed error paths in TARGET_SYSLOG_ACTION_READ_ALL to match

http://lxr.free-electrons.com/source/kernel/printk/printk.c?v=4.7#L1335

Should fix also the build error in:

https://lists.gnu.org/archive/html/qemu-devel/2016-10/msg03721.html

Signed-off-by: Aleksandar Markovic <aleksandar.markovic@imgtec.com>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
