libbb: introduce and use BB_EXECVP_or_die()

libbb: introduce and use BB_EXECVP_or_die()

function                                             old     new   delta
BB_EXECVP_or_die                                       -      47     +47
time_main                                           1042    1043      +1
chrt_main                                            371     364      -7
ionice_main                                          292     282     -10
setsid_main                                           69      56     -13
nohup_main                                           236     223     -13
cttyhack_main                                        266     253     -13
chroot_main                                           94      81     -13
chpst_main                                           746     733     -13
timeout_main                                         297     279     -18
taskset_main                                         541     522     -19
vfork_child                                           67      45     -22
parse                                                975     953     -22
lpd_main                                             770     748     -22
launch_helper                                        192     170     -22
tcpudpsvd_main                                      1810    1782     -28
nice_main                                            190     156     -34
env_main                                             242     206     -36
run_command                                          221     174     -47
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 1/17 up/down: 48/-352)         Total: -304 bytes

Signed-off-by: Pascal Bellard <pascal.bellard@ads-lu.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>