Dpkg::Vendor::Ubuntu: Fix lto feature to honor DEB_BUILD_OPTIONS

The recently added lto handling for Ubuntu, did not take into account
that the DEB_BUILD_OPTIONS and DEB_BUILD_MAINT_OPTIONS environment
variables are parsed and their options applied just after setting the
defaults, and before applying any arch-specific mask.

We add a new init_build_features() internal method, which sits between
these two actions, so that the Ubuntu vendor module can modify the
defaults before any user or maintainer override are applied.

Fixes: commit df7627ac
Fixes: https://bugs.launchpad.net/bugs/2002582