Be more selective when filtering for lib*.so.N files.  These are deleted
at the end of the upgrade process, after warning users to upgrade any
3rd party software (e.g., from the ports tree) which might link to the
libraries being removed.

Prior to this commit, the line
  /usr/lib/libc.so|...|/lib/libc.so.7
matched the regex, which -- upgrading from 9.x to 10.x, where libc.so is
a regular file and thus was not part of a line which matched the regex --
resulted in freebsd-update thinking that /usr/lib/libc.so was a shared
library which was being removed as part of the upgrade.  This had some
unfortunate consequences.

This will be part of an upcoming Errata Notice.