logwrap: convert to C++, rename function logwrap_fork_execvp().

Call the function now logwrap_fork_execvp() and fix const correctness
issues, so no longer do callers need to const_cast<> their args.

Test: logwrapper still works
Change-Id: Iea34f5cae90a06a37d395bf9a91e01fb38c35fa6