syslogd: add option to log to Linux kernel printk buffer

Why invent our own shared memory circular buffer when the kernel has a
perfectly fine one already?

This can be used as a smaller/simpler alternative to the syslogd IPC support
(as IPC shmem/klogd/logread aren't needed), while also allowing centralised
logging of everything (kernel messages, userspace bootup and syslog)
when used together with ttyprintk.

Notice that kernel 3.5+ is needed to store syslog facility in printk buffer,
otherwise only the priority is stored.

bloat-o-meter compared to IPC+klogd+logread:
function                                             old     new   delta
get_linux_version_code                                 -      84     +84
lbb_prepare                                           25      90     +65
applet_nameofs                                         6       -      -6
static.stdout@@GLIBC_2                                 8       -      -8
applet_names                                          23       9     -14
bb_msg_standard_output                                16       -     -16
init_sem                                              18       -     -18
xatoull_range                                         19       -     -19
overlapping_strcpy                                    21       -     -21
init_data                                             56      32     -24
applet_main                                           24       -     -24
main                                                 124      99     -25
full_write2_str                                       26       -     -26
error_exit                                            26       -     -26
bb_basename                                           30       -     -30
sem_up                                                32       -     -32
interrupted                                           35       -     -35
fflush_stdout_and_exit                                38       -     -38
bb_banner                                             46       -     -46
find_applet_by_name                                   59       -     -59
bb_signals_recursive_norestart                        90       -     -90
run_applet_no_and_exit                               104       -    -104
timestamp_and_log                                    651     523    -128
syslogd_main                                         798     581    -217
xstrtoull_range_sfx                                  267       -    -267
run_applet_and_exit                                  432       -    -432
klogd_main                                           490       -    -490
logread_main                                         508       -    -508
.rodata                                             1870     937    -933
bb_common_bufsiz1                                   8193       -   -8193
------------------------------------------------------------------------------
(add/remove: 2/26 grow/shrink: 1/6 up/down: 149/-11829)    Total: -11680 bytes

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Merge git://busybox.net/busybox