installworld: handle ldd including preloaded objects

The installworld target makes a temporary copy of binaries to be used
during the install. Libraries that they depend on are also included,
found by using ldd.

After commit 0913953c9ed0 ldd started listing preloaded objects,
including [vdso], under a [preloaded] header. Skip ldd output that is
enclosed in square brackets.

Reviewed by: cy, kib [earlier version]
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34734

(cherry picked from commit b3b462229f972e2ed24d450d7d2f8855cdd58a87)