libbpf-tools: fix EINTR related issues

libbpf-tools: fix EINTR related issues

1. Most of the tools that use perf_buffer__poll() were not handling the
case when it was interrupted by a signal, they were just ending.
We noticed this issue by running the tools inside a container, after
some seconds they will finish:

```
$ time /execsnoop
...
runc             210198 939      0 /usr/sbin/runc --version
docker-init      210205 939      0 /usr/bin/docker-init --version
Error polling perf buffer: -4

real	0m48.913s
user	0m0.020s
sys	0m0.033s
```

This commit fixes that by checking if errno is EINTR after calling
perf_buffer__poll().

2. Many tools were returning non zero when ended by SIG_INT.

```
$ sudo ./execsnoop
PCOMM            PID    PPID   RET ARGS
runc             203967 939      0 /usr/sbin/runc --version
docker-init      203973 939      0 /usr/bin/docker-init --version
calico           203974 724      0 /opt/cni/bin/calico
portmap          203985 724      0 /opt/cni/bin/portmap
bandwidth        203990 724      0 /opt/cni/bin/bandwidth
^C
$ echo $?
130
```

3. Some tools were missing the SIG_INT handler

Signed-off-by: Mauricio Vásquez <mauricio@kinvolk.io>