libbpf tcptop: Fix PID column alignment when pids are large

[bernhardkaindl]

Fix PID column alignment when PIDs are large in libbpf-tools/tcptop:

Check the maximum width of the PIDs on the system and adjust the format to avoid column misalignment:

PID     COMM         LADDR                 RADDR                  RX_KB  TX_KB
1987802 wget         127.0.0.1:44960       127.0.0.1:443              6      0
1987815 wget         127.0.0.1:44968       127.0.0.1:443              6      0
2770    stunnel      127.0.0.1:51460       127.0.0.1:80               4      0
2770    stunnel      127.0.0.1:51452       127.0.0.1:80               4      0
1936412 sshd         10.71.56.137:22       10.71.8.14:45682           0      2
1987805 curl         127.0.0.1:44964       127.0.0.1:443              1      0
1987818 curl         127.0.0.1:44972       127.0.0.1:443              1      0
2770    stunnel      127.0.0.1:51456       127.0.0.1:80               0      0
2770    stunnel      127.0.0.1:51464       127.0.0.1:80               0      0
1918977 sshd         10.71.56.137:22       10.71.8.14:51046           0      0

• Also fix the alignment of the "TX_KB" label by moving the trailing "\n" character from it to the format string
• Also add libbpf-tools/tcptop_example.txt documenting the example.

PS: It seems that GitHub CI fails on a few test cases (flaky), not caused by this PR.