udhcpc: support resolv.conf symlinks

Often it is desirable to have /etc/ be on read-only storage (well, the
whole rootfs) but have things like /etc/resolv.conf be symlinks to a
writable location.  Tweak the simple script to support that.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
udhcpc: tweak math shell style with the metric var

Some shells (like dash) are lame and omit the POSIX increment/decrement
feature (because it is listed as optional).  Tweak the shell script to
work in all POSIX variants.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
mdev: improve $SEQ handling; improve debug logging

Sequential run of concurrent mdev's was too simplistic:
they waited for /dev/mdev.seq to match. This could sometimes
cause cumulative loss of time on the order of a second.

Added SIGCHLD signaling from exiting mdev to all other mdev's.
Added debugging required to see that code actually works as intended.

Example of /dev/mdev.log (with "woken up" elevated from dbg lvl 3 to 2):

mdev[1023]: first seq written
     ^^^^ seq, not pid
mdev[1023]: 35.022395 ACTION:add SUBSYSTEM:module DEVNAME:(null) DEVPATH:/module/lib80211
mdev[1023]: rule matched, line -1
                          ^^^^^^^ means "default rule"
mdev[1023]: 35.022676 exiting
            ^^^^^^^^^ second,usec timestamp
mdev[1024]: 35.069691 ACTION:add SUBSYSTEM:vc DEVNAME:vcs9 DEVPATH:/devices/virtual/vc/vcs9
mdev[1024]: dev 7,9
mdev[1025]: 35.069889 waiting for '1024'
mdev[1026]: 35.069946 waiting for '1024'
mdev[1027]: 35.070151 waiting for '1024'
mdev[1024]: rule matched, line -1
mdev[1024]: mknod vcs9 (7,9) 20660 0:0
mdev[1024]: 35.070346 exiting
mdev[1025]: woken up
mdev[1026]: woken up
mdev[1025]: 35.071213 ACTION:add SUBSYSTEM:vc DEVNAME:vcsa9 DEVPATH:/devices/virtual/vc/vcsa9
            ^^^^^^^^^ took only a millisecond to start running after prev mdev exited
mdev[1025]: dev 7,137
mdev[1027]: woken up
mdev[1025]: rule matched, line -1
mdev[1025]: mknod vcsa9 (7,137) 20660 0:0
mdev[1025]: 35.072109 exiting

function                                             old     new   delta
mdev_main                                            849    1372    +523
curtime                                                -      59     +59
dirAction                                             87     134     +47
static.ts                                              -       8      +8
keywords                                              19      12      -7
make_device                                         2189    2119     -70

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
Fix move_to_unaligned16

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
Merge git://busybox.net/busybox