Avoid job control messages when launching GDB

It is my understanding that the following shell script is the cause of microsoft/vscode-cpptools#3298:

https://github.com/microsoft/MIEngine/blob/9470e98d946772ed1ace9ab68e9fbd4d88090b42/src/MICore/UnixUtilities.cs#L49-L63

That is the annoying job control message in the terminal window, for example:

[1] + Done /usr/bin/gdb --interpreter=mi --tty=${DbgTerm} 0</tmp/Microsoft-MIEngine-In-ndptacu6.2ri 1>/tmp/Microsoft-MIEngine-Out-9278nini.hzx

By taking a look at the above script I noticed that GDB is sent to the background just for the sake of fetching its own PID so that it can be written to file. Then the GDB process is waited using wait and in order to do so the shell job control mechanism needs to be enabled.

The unfortunate drawback is the above issue. I'm not fully aware of the internals nor why it is important to save the PID to file but the following provides a way to obtain the same behaviour without the annoying job control message (which is basically what already happens in macOS):

set -o monitor

sleep 3 &
pid=$!
echo "save pid $pid to file..."
fg >/dev/null

The output is:

$ sh test.sh
save 47847 to file...

Moreover, the above comments about Linux vs macOS don't really make sense to me, but anyway this works in both OSs, both sh and bash and both in a shell script and inline:

$ sh -c 'set -o monitor; sleep 3 & pid=$!; echo "save $pid to file..."; fg >/dev/null'
save 47881 to file...

Is this something that are you willing to fix?

Start with src/MICore/UnixUtilities.cs lines 49-63, where the shell script launches GDB and records its PID. Reproduce the job-control message using the issue's shell examples, then verify that the launch still records and waits for GDB without printing that message on Linux and macOS.