Enumerate processes/devices and show help with r2 frida://?

[trufae]

[mrmacete]

One possible way could be:

• frida://* list processes on host
• frida://*/ lists devices
• frida://DEVICE_ID/* list processes on a particular device
• frida://*/* list processes on the first device
• frida://*/Safari attaches to safari on the first device
• frida://? show this help

[trufae]

related to https://github.com/nowsecure/r2frida/issues/123