bin: support hook overrides in xt-customize-image

The systems administrator can optionally provide site-specific
revisions of upstream hooks by placing them in the directory
/etc/xen-tools/hooks.d/ as a method for disabling hooks, adding
site-specific extra features that don't get overwritten on upgrades,
or patching bugs early.

Signed-off-by: Adrian C. (anrxc) <anrxc@sysphere.org>
Update d/changelog + d/dirs for Adrian C.'s patch
Ignore cover_db directory in t/perl-syntax.t