i18n: reword extraction of voices texts

Instead of grepping the intro text, and manipulating the output of grep,
use an awk script to do the extraction and printing:

• slightly cleaner solution
• easier way to extract (and print) the actual intro text
• it is possible to output the file+line location of each string

Also, output the result to a rc.cpp file, so repack-pot can do its job
nicely without complaining about half-baked location comments found in
the pot file.