Redirect and assign before forking in simple command

[magicant]

POSIX says:

If the command name is not a special built-in utility or function, the variable assignments shall be exported for the execution environment of the command and shall not affect the current execution environment except as a side-effect of the expansions performed in step 4.

Existing shells (including C yash but not zsh) seem to allow such a side-effect to affect the current environment. For example:

a=${b=x} no_such_utility
echo a=$a b=$b

This script prints a= b=x.

To support this behavior, the shell has to perform redirections and assignments before forking a subshell.

[magicant]

Probably we should revert 7bd19e9.