Unterminated quoted string on Line 59

I'm getting the following error for all invocations of `pmonitor.sh` (including `test-pmonitor.sh`):

```
/bin/sh: 1: Syntax error: Unterminated quoted string
```

I've isolated the problem to [line 59](https://github.com/dspinellis/pmonitor/blob/master/pmonitor.sh#L59)

```
"ls -l '\''" fname "'\'' 2>/dev/null" | getline
```

This is probably a `mawk` version compatibility issue; I had the same problem as #7 but the [fix there](https://github.com/dspinellis/pmonitor/issues/7#issuecomment-413026440) seemed to work for me.

## Environment
```
$ lsb_release -ds
Ubuntu 18.04.2 LTS
$ awk -W version
mawk 1.3.3 Nov 1996, Copyright (C) Michael D. Brennan

compiled limits:
max NF 32767
sprintf buffer 1020
```

Start by reproducing the failure with pmonitor.sh and test-pmonitor.sh on the reported Ubuntu 18.04.2 environment using mawk 1.3.3, then inspect line 59 and the related fix discussed in issue #7. Done means both scripts run without the unterminated quoted string error.

Written by the indexing model from the issue text.