udhcpd: add -f "foreground" and -S "syslog" options

udhcpd: add -f "foreground" and -S "syslog" options
udhcpc: add -S "syslog" options
remove CONFIG_FEATURE_UDHCP_SYSLOG

function                                             old     new   delta
udhcpd_main                                         1168    1201     +33
udhcpc_main                                         2414    2425     +11
static.udhcpc_longopts                               191     200      +9
qgravechar                                           112     108      -4
packed_usage                                       22843   22827     -16
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 3/2 up/down: 53/-20)             Total: 33 bytes

git-svn-id: svn://busybox.net/trunk/busybox@19507 69ca8d6d-28ef-0310-b511-8ec308f3f277