libbpf-tools/memleak: Change the start message to match memleak python

[ekyooo]

Change the start message to match memleak python,
make it concise, and include the PID.

Before:

  # ./memleak
    using default object: libc.so.6
    using page size: 4096
    tracing kernel: true
    Tracing outstanding memory allocs...  Hit Ctrl-C to end

  # ./memleak -p `pidof alloctest`
    using default object: libc.so.6
    using page size: 4096
    tracing kernel: false
    Tracing outstanding memory allocs...  Hit Ctrl-C to end

After:

  # ./memleak
    using default object: libc.so.6
    using page size: 4096
    Attaching to kernel allocators, Ctrl+C to quit.

  # ./memleak -p `pidof alloctest`
    using default object: libc.so.6
    using page size: 4096
    Attaching to pid 29973, Ctrl+C to quit.