add recursive rpath support to dynamic linker

add recursive rpath support to dynamic linker

previously, rpath was only honored for direct dependencies. in other
words, if A depends on B and B depends on C, only B's rpath (if any),
not A's rpath, was being searched for C. this limitation made
rpath-based deployment difficult in the presence of multiple levels of
library dependency.

at present, $ORIGIN processing in rpath is still unsupported.