ash: [REDIR] Replace copyfd by savefd and use dup2 elsewhere

ash: [REDIR] Replace copyfd by savefd and use dup2 elsewhere

Upstream commit:

    Date: Sat, 12 May 2007 18:00:57 +1000
    [REDIR] Replace copyfd by savefd and use dup2 elsewhere

    There are two kinds of users to copyfd, those that want to copy an fd to
    an exact value and those that want to move an fd to a value >= 10.  The
    former can simply use dup2 directly while the latter share a lot of common
    code that now constitutes savefd.

This does not change much, just reducing our divergence from dash code.

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