hush: kill builtin and kill %jobspec support

hush: kill builtin and kill %jobspec support

Also made it and printf, type and wait builtins optional.

function                                             old     new   delta
builtin_kill                                           -     323    +323
bltins1                                              336     348     +12
builtin_type                                         114     116      +2
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(add/remove: 1/0 grow/shrink: 2/0 up/down: 337/0)             Total: 337 bytes

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