init: don't use fixed size buffer for command

init: don't use fixed size buffer for command

We store init actions forever. 256 bytes per action means that
a typical inittab of ~10 commands uses 2.5k just to remember
command strings - which are usually _much_ shorter than 256 bytes.

At a cost of a bit more code, it's possible to allocate
only actually needed amount.

function                                             old     new   delta
init_exec                                            224     248     +24
new_init_action                                      140     142      +2

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