Permit special field in paths passed to OS_CLI
OS_CLI permits commands to be prefixed with a temporary filing
system, for example ADFS:Format
It also passes commands unrecognised to FileSwitch to be executed,
so ADFS::HardDisc4.$.!Boot first sets the temporary filing to
ADFS then executes :HardDisc4.$.!Boot.
This does not work if the path contains a special field, currently this results in an error as the special filed would be lost.
Unfortunately many programs, including the RISC OS source fail because of this.
Instead if a filing system prefix with special field
is found switch to the Temporary filing system and
treat the whole command as a path to execute. For example
passing IXFS#W:$.HardDisc4.!Boot sets the temporary filing
system to IXFS then executes IXFS#W:$.HardDisc4.!Boot.