partcl: Improved interactive mode

In real tcl, you can use interactive mode to run commands like a shell; There's now just enough logic in partcl to do this as well.

Most of the infrastructure for this was already in place; just added a minimal version of

[file executable] - only checks executable status for other, not group or owner.
[file split] - isn't windows-friendly, nor does it deal with potentially confusing chars like '~'
[exec] - ignores all of the shell metachars and just invokes parrot's spawnw opcode as is.

$ ~/bird/bin/parrot tcl.pbc

% ls src

binary.c binary.h binary.o class grammar macros.pir mathops.pir ops pmc returncodes.pasm tclsh.pir

% rm src/binary.o

% ls src

binary.c binary.h class grammar macros.pir mathops.pir ops pmc returncodes.pasm tclsh.pir

Most of the heavy lifting is done by init.tcl from the standard library.

partcl