Better parsing of invalid @-codes

@-codes never start with a number (decimal digit)
@-codes never contain any whitespace (including tabs, CR and LF).

These 2 issues caused the stock batch file transfer menu for RIP terminals
to display all messed-up.

This RIP menu (text/menu/batchxfr.rip) contains @s and they triggered some
stripping of text and expanding to a text.dat string (!).