freebsd-update: handle file -> directory on upgrade

freebsd-update: handle file -> directory on upgrade

Upgrading from FreeBSD 13.2 to 14.0 failed with
  install: ///usr/include/c++/v1/__string exists but is not a directory
because __string changed from a file to a directory with an LLVM
upgrade.

Now, remove the existing file when the type conflicts.  Note that this
is only an interim fix to facilitate upgrades from 13.2 for 14.0 BETA
testing.  This change does not handle the directory -> file case and
further work is needed.

PR:		273661
Reviewed by:	dim, gordon
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D41893