rtld: Implement ELF filters (DT_FILTER and DT_AUXILIARY)

rtld: Implement ELF filters (DT_FILTER and DT_AUXILIARY)

Filtrees are loaded on demand unless LD_LOADFLTR environment variable is
set or -z loadfltr was specified during the linking.  This forces rtld
to upgrade read-locked rtld_bind_lock to write lock when it encounters an
object with filter during symbol lookup.

Consolidate common arguments of the symbol lookup functions in the SymLook
structure.

Taken-from: FreeBSD SVN 216728 (27 DEC 2010)