dhcpc: code shrink in good_hostname

dhcpc: code shrink in good_hostname

Incorporated valid_domain_label into good_hostname to simplify the implementation.

function                                             old     new   delta
static.xmalloc_optname_optval                        973     958     -15
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-15)             Total: -15 bytes
   text	   data	    bss	    dec	    hex	filename
 993144	  16915	   1872	1011931	  f70db	busybox_old
 993129	  16915	   1872	1011916	  f70cc	busybox_unstripped

Signed-off-by: Martin Lewis <martin.lewis.x84@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>