Added options for gdb/valgrind

https://jira.esss.lu.se/browse/ICSHWI-4848 requests that iocsh.bash have an option to use valgrind. This MR essentially copies the method used in the iocsh script to allow both gdb and valgrind to be triggered from the command line.

This obviates the need to have the separate iocsh_gdb.bash, but I have not chosen to delete that yet, as other people may be using it.