execsnoop: don't print newlines in argv

by escaping newlines. Fixes #1037

* Before:
```
$ sudo /usr/share/bcc/tools/execsnoop
PCOMM            PID    PPID   RET ARGS
awk              9910   7831     0 /usr/bin/awk
BEGIN { print "hi" }
```

* With this patch:
```
$ sudo /usr/share/bcc/tools/execsnoop
PCOMM            PID    PPID   RET ARGS
awk              10033  7831     0 /usr/bin/awk \nBEGIN { print "hi" }
```