netcat help output in error log when running xtrabackup SST

The feature check for "ncat" in the XtraBackup SST scripts only works on CentOS 7, on other distributions it leads to full netcat help output being written to the error log instead.

This is due to the nc version in CentOS 7 writing help output to stdout, while the version in CentOS 6, and in Ubuntu 16.04, write to stderr instead.

Redirecting stderr to stdout with 2>&1 should make the check work for all versions of netcat:

if nc -h 2>&1 | grep -q ncat;then

instead of current

if nc -h | grep -q ncat;then

The feature check for "ncat" only works on CentOS 7, on other distributions it leads to full netcat help output being written to the error log instead.

This is due to the nc version in CentOS 7 writing help output to stdout, while the version in CentOS 6, and in Ubuntu 16.04, write to stderr instead.