libbpf javagc fails when no PID is provided

[jeromemarchand]

The libbpf javagc tool fails with no useful error message when no PID is provided even though the "-p" option is described as optional.
A quick look at the code let me think that the tool is only meant to trace one PID only and thus the option should have been made mandatory, and the tool should have failed at the parsing stage, with a proper error message.
As a side note, the tool help message is lacking compared to the equivalent bcc tool.