Declaration utility

[magicant]

(Issue migrated from https://osdn.net/projects/yash/ticket/46042)

The notion of "declaration utility" is going to be added in the next version of POSIX.

• https://www.austingroupbugs.net/view.php?id=351
• https://www.austingroupbugs.net/view.php?id=1393
• https://www.austingroupbugs.net/view.php?id=1535

We need to implement it when the standard is finalized.

Considerations:

• [ ] Should declaration utilities be recognized at parse-time or run-time?
• [ ] How do assignments in arguments interact with normal assignments? (e.g. a=foo export a=bar)
• [ ] Should we support array assignment like export a=(1 2 3)? How?