libdtrace: Don't use a read-only handle for enumerating pid probes.

libdtrace: Don't use a read-only handle for enumerating pid probes.

Enumeration of return probes involves disassembling subroutines in the
target process, and ptrace(2) is currently used to read from the target
process. libproc could read from the backing file instead to avoid this
problem, but in the common case libdtrace will have a writeable handle
on the process anyway. In particular, a writeable handle is needed to list
USDT probes, and libdtrace will cache such a handle for processes that it
controls via dtrace -c and -p.