start_batch_download() now creates a list of the filenames and passes as %s to protocol program (for Unix sz support).

Win32 external() now uses the comspec if the EX_SH mode bit is set.
Unix external() now supports the EX_SH mode bit and auto-redirection detection.

Changed EX_CC to EX_SH.

Changed bi-dir xfer commands to chdir to the temp_dir.

Fixed Telnet IAC (for binary tranfsers) and LF-expansion (for stdio doors) for Unix externals, using pipes.

Use checkfname() to make sure the user doesn't delete any files in other dirs.

Cleaned up the cleanup==1 case a bit.

Better detection of corrupted SBL database entries.