tar: conditionally don't wait for vforked child to exec, as it always

tar: conditionally don't wait for vforked child to exec, as it always
works right on Linux, and anyway mayresult only on less-than-clear error
message only, it will not cause tar to misbehave.

function                                             old     new   delta
open_transformer                                      98      80     -18
writeTarFile                                         714     547    -167
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-185)           Total: -185 bytes
   text    data     bss     dec     hex filename
 770651    1051   10764  782466   bf082 busybox_old
 770463    1051   10764  782278   befc6 busybox_unstripped