dblink: case insensitive support for bug #524236

This adds dblink._contents_contains, _contents_keys, and
_contents_unmap_key methods that provide an interface for contents
operations with "implicit" case handling. The new methods are
implemented in a separate ContentsCaseSensitivityManager class,
in order to avoid adding more bloat to vartree.py.

X-Gentoo-Bug: 524236
X-Gentoo-Url: https://bugs.gentoo.org/show_bug.cgi?id=524236
FEATURES=case-insensitive-fs for bug #524236

When case-insensitive-fs is enabled in FEATURES, the dblink.isowner
method, _owners_db class, and ConfigProtect class will be
case-insensitive. This causes the collision-protect and unmerge code
to behave correctly for a case-insensitive file system. If the file
system is case-insensitive but case-preserving, then case is preserved
in the CONTENTS data of installed packages.

X-Gentoo-Bug: 524236
X-Gentoo-Url: https://bugs.gentoo.org/show_bug.cgi?id=524236