seccomp: only abort on syscall name resolution failures (#3701)

seccomp: only abort on syscall name resolution failures (#3701)

seccomp_syscall_resolve_name() can return a mix of positive and negative
(pseudo-) syscall numbers, while errors are signaled via __NR_SCMP_ERROR.
This commit lets the syscall filter parser only abort on real parsing
failures, letting libseccomp handle pseudo-syscall number on its own
and allowing proper multiplexed syscalls filtering.