Merged from trunk 5305:5318

Merged from trunk 5305:5318

remove pointless redirects to /dev/null when using hasq() (SpankMan)

force environment.bz2 to be clobbered if it already exists (SpankMan)

In depgraph.altlist(), pop as many leaf nodes as possible when no
relationship has been ignored.  This can halve the time taken for a
large merge order calculation.

Don't optimize depgraph.altlist() in reversed mode since the
optimization destroys --tree ouput.

Add --debug support for --depclean.

Do some minor cleanup of the --depclean code.

Make depclean skip empty depstrings.

Disable the spinner when --debug is enabled so that it doesn't
interfere.

Catch PortageException from portdbapi.aux_get since it can happen when
gpg is in FEATURES.

For bug #158354, make "No" the default response for unmerge actions.

svn path=/main/branches/prefix/; revision=5320