bpf: Make bytecode-file reading a little more robust

bpf: Make bytecode-file reading a little more robust

bpf_parse_string() will now correctly handle:

- Extraneous whitespace,
- OPs on multiple lines and
- overlong file names.

The added feature of allowing to have OPs on multiple lines (like e.g.
tcpdump prints them) is rather a side effect of fixing detection of
malformed bytecode files having random content on a second line, like
e.g.:

| 4,40 0 0 12,21 0 1 2048,6 0 0 262144,6 0 0 0
| foobar

Cc: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Phil Sutter <phil@nwl.cc>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>