Dynamic root: Move ld-elf.so.2 to /libexec

Dynamic root: Move ld-elf.so.2 to /libexec

* Install it first to avoid breaking upgrades for systems previously
  having it located in /usr/libexec. install(1), beeing a dynamic binary,
  needs it at runtime.

* Since ld-elf.so.2's path is embedded in ELF binaries, also create a
  symbolic link in the old location to support existing binaries.

* Code to handle chflags attributes obtained from FreeBSD.