libbpf: Stringify errno in log messages in libbpf.c

Convert numeric error codes into the string representations in log
messages in libbpf.c.

Signed-off-by: Mykyta Yatsenko <[email protected]>
Signed-off-by: Andrii Nakryiko <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

Author: mykyta5