seccomp: Use -1 marker for end of mode 1 syscall list

The terminator for the mode 1 syscalls list was a 0, but that could be
a valid syscall number (e.g. x86_64 __NR_read). By luck, __NR_read was
listed first and the loop construct would not test it, so there was no
bug. However, this is fragile. Replace the terminator with -1 instead,
and make the variable name for mode 1 syscall lists more descriptive.

Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Will Drewry <wad@chromium.org>
Signed-off-by: Kees Cook <keescook@chromium.org>