ash: make shellexec capable of using separate argv[0] and filename to exec

ash: make shellexec capable of using separate argv[0] and filename to exec

function                                             old     new   delta
execcmd                                               71      78      +7
shellexec                                            221     224      +3
evalcommand                                         1158    1161      +3
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(add/remove: 0/0 grow/shrink: 3/0 up/down: 13/0)               Total: 13 bytes

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