helper: use generic extra files interface

This replaces the pipes object and integrates context into helper process lifecycle.

Signed-off-by: Ophestra <cat@gensokyo.uk>

helper/seccomp/seccomp-export.h

@@ -20,5 +20,4 @@ typedef enum {
 } f_syscall_opts;
 
 extern void F_println(char *v);
-int f_tmpfile_fd();
 int32_t f_export_bpf(int fd, uint32_t arch, uint32_t multiarch, f_syscall_opts opts);