installworld: handle ldd including preloaded objects

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)