Add SetPrintLevel to support changing print level of libbpf messages

[mozillazg]

Closes #6

Depend on #270

[mozillazg]

Ready for review.

A demo for this function: mozillazg#6

$ sudo ./main-static
libbpf: loading main.bpf.o
libbpf: elf: section(3) license, size 13, link 0, flags 3, type=1
libbpf: license of main.bpf.o is Dual BSD/GPL
libbpf: elf: section(4) .maps, size 32, link 0, flags 3, type=1
libbpf: elf: section(9) .BTF, size 470, link 0, flags 0, type=1
libbpf: elf: section(13) .symtab, size 168, link 1, flags 0, type=2
libbpf: looking for externs among 7 symbols...
libbpf: collected 0 externs total
libbpf: map 'numbers': at sec_idx 4, offset 0.
libbpf: map 'numbers': found type = 1.
libbpf: map 'numbers': found key [8], sz = 4.
libbpf: map 'numbers': found value [8], sz = 4.
libbpf: map 'numbers': found max_entries = 5.
libbpf: map 'numbers': created successfully, fd=8

[geyslan]

@mozillazg Thanks for this PR and sorry for the delay in getting to it.

This seems awesome, but as we've considered to publicise an interface to callbacks, perhaps this has become old for analysis. Would you mind of reviewing #284. Then, if you agree, I would close this.

[geyslan]

Then, if you agree, I would close this.

Or, if you do prefer, you can tackle SetPrintLevel on top of #284 or wait it get into and rebase this.