RepoConfigLoader: pass source arg to read_file

The 'source' keyword argument is needed since otherwise ConfigParser
may throw a TypeError because it assumes f.name is a native string
rather than binary when constructing error messages.
RepoConfigLoader: handle configparser.Error
RepoConfigLoader: return early for parse error

The configparser state is unreliable (prone to odd quirky exceptions)
after is has thrown an error, so just return early.
RepoConfigLoader: initialize missing_repo_names
RepoConfigLoader: handle invalid repos.conf more
RepoConfig: safe config for repoman/egencache

For repoman/egencache, omit local configuration overrides involving
aliases, eclass-overrides, and masters.
RepoConfigLoader: handle equal priorities

Include repo.name in sort key, for predictable sorting even when
priorities are equal.
RepoConfigLoader: fix last commit
depgraph: fix --exclude/--newuse interaction

This fixes a case where an installed package could possibly be rejected
due to --newuse, even though it was selected by --exclude. The problem
may have been triggered randomly by differences in graph traversal
order, which may have been affected by hash randomization (enabled by
default since Python 3.3).
depgraph: fix installed/newuse logic more

The matched_pkgs_ignore_use logic, introduced in commit
dc8d70479dfb54a1f2561ba3fe82d7a62d97964f, is questionable because we
need to ensure that an installed package can be selected if there are
no other available packages that match a given USE-dep.
Fix cross-prefix bugs, and test.

* Fix create_trees to correctly handle the case where ROOT=/ for two
  different EPREFIX offsets.
* For best/has_version, handle the case where ROOT=/ for bug prefixes,
  though each has a different EPREFIX offset.
* Use portage.const.EPREFIX for default EMERGE_LOG_DIR offset, which
  is analogous to existing un-prefixed behavior.
* Use portage.const.EPREFIX for default PORTAGE_CONFIGROOT, which is
  analogous to existing un-prefixed behavior.
* Add cross-prefix cases to emerge tests, including chpathtool for
  binpkgs.
Test cross-prefix best/has_version --host-root.
emirrordist: wget --no-check-certificate

Use --no-check-certificate since Manifest digests should provide
enough security, and certificates can be self-signed or whatnot.