sh(1): Extend documentation about subshells.

sh(1): Extend documentation about subshells.

Because sh executes commands in subshell environments without forking in
more and more cases, it makes sense to describe subshell environments
more precisely using ideas from POSIX, together with some
DragonFly-specific items.

In particular, the hash and times builtins may not behave as if their
state is copied for a subshell environment while leaving the parent
shell environment unchanged.

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