bpf: move definitions of bpf_attr structures to separate header file

bpf: move definitions of bpf_attr structures to separate header file

This also unifies decoders of bpf commands by moving common code
to new macros.

* bpf_attr.h: New file.
* Makefile.am (strace_SOURCES): Add it.
* bpf.c: Include it.
(DEF_BPF_CMD_DECODER): Rename to BEGIN_BPF_CMD_DECODER, add code common
to all decoders.  All users updated.
(END_BPF_CMD_DECODER): New macro, add its invocation to all users
of BEGIN_BPF_CMD_DECODER macro.