add relative path support to .drpath files

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on February 14, 2013 23:37:25_

xref issue #955 this covers adding relative path support a la $ORIGIN

xref my note at the time:
but we'll wait until we have more concrete scenarios on client deployment:
we don't control the client layout vs DR or 3rd-party libs so we may have
to give more control to the client. so for now we stick with just
absolute paths. more can be added incrementally. we'll file future issues
as we need them.

the current usage is for installing the strace client which uses drsyscall.dll.

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=1078_