man: bpf: also mention that bpfc can be used for seccomp-BPF

Seccomp-BPF is used for syscall sandboxing. bpfc can also be used for
creating such filters without any problems.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
man: bpfc: add example of loadable x86_64 seccomp-BPF filter

Add the example in bpfc code from http://outflux.net/teach-seccomp/
to the man page, so that people can also experiment with seccomp
bpf and to demonstrate that there are no limits on bpfc when it
comes to this. Actually this code is even more efficient than the
one given in the example URL above.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
bpfc: stress that also seccomp-BPF can be generated with bpfc

Just add a minor sentence to help stating this.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
docs: readme: also stress bpf seccomp in the README page

We should also make this clear from the beginning that one can use
bpfc for seccomps as well. Might be interesting for security
researchers to play around with.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
bpf: very minor style fixups and paths marked unlikely

Mark some error paths as unlikely, and fixup some minor whitespace
issues. Nothing big to see here.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>