support symbolic substitution in config file

[prjemian]

With some form of symbolic substitution, this code:

[amb-vm]
title = IOC: prj on amb-vm
1 medm prj = /home/mintadmin/bin/start_medm_prj.sh
2 separator =
3 start IOC = cd  /usr/local/epics/ioc/prj/iocBoot/iocLinux; ./prj.sh start
10 separator =
14 IOC console = cd  /usr/local/epics/ioc/prj/iocBoot/iocLinux; mate-terminal -t "XXX IOC console" -e "./prj.sh console"
15 IOC status = cd  /usr/local/epics/ioc/prj/iocBoot/iocLinux; ./prj.sh status
20 separator =
26 stop IOC = cd  /usr/local/epics/ioc/prj/iocBoot/iocLinux; ./prj.sh stop

might become

[amb-vm]
title = IOC: prj on amb-vm
symbol HOME = /home/mintadmin
symbol IOCDIR = /usr/local/epics/ioc/prj/iocBoot/iocLinux
symbol BINDIR = HOME/bin
symbol TERMINAL = mate-terminal
1 medm prj = BINDIR/start_medm_prj.sh
2 separator =
3 start IOC = cd IOCDIR; ./prj.sh start
10 separator =
14 IOC console = cd IOCDIR; TERMINAL -t "XXX IOC console" -e "./prj.sh console"
15 IOC status = cd IOCDIR; ./prj.sh status
20 separator =
26 stop IOC = cd IOCDIR; ./prj.sh stop

Should be scoped within the section of the .ini file