Make linux_ptrace_attach_fail_reason return an std::string
This patch makes linux_ptrace_attach_fail_reason and
linux_ptrace_attach_fail_reason_string return std::string. It also
replaces usages of struct buffer with std::string. This allows getting
rid of a cleanup in in linux_ptrace_attach_fail_reason_string and
simplifies the code in general.
Something that looks odd to me is that in
linux_ptrace_attach_fail_reason, if the two messages are appended, there
is no separating space or \n, so the result won't be very nice. I left
it as-is for now though.