Legacy profiles that are not based on interrupt-based sampling
only include addresses that are call stack return addresses. For
them, all callstack addresses should be adjusted by -1 to land
on top of the call instruction and permit accurate symbolization.

This fixes issue #9

The legacy profilez parser handles duplicate leaf samples that are a
common artifact of satck unwinding. Apply the same technique to threadz
profiles where duplicate samples also occur.

A Go redefinition error causes pprof to skip parsing profile mappings.

Also update the build instructions to use go get