Add option for custom log string to bpf_prog_load

Add option for custom log string to bpf_prog_load

bpf_prog_load was unconditionally printing the error from the kernel to
stderr. If libbpf is used in a library, that library may want to collect
the error to a custom location (a logfile). So, provide an option to
store the string elsewhere than stderr.

Signed-off-by: Brenden Blanco <bblanco@plumgrid.com>