[BRLTTY] Search for command feature.

[Stéphane Doyon]

I often search for text on the screen. Currently the only way to specify 
it is to cut the target text from somewhere (as in cut&paste). So I often 
find myself switching to a different virtual terminal, typing in a string 
(usually at a command prompt, even though the string isn't a command), 
cutting it, clearing the input, switching back to the original virtual 
terminal, and then searching.

That's clunky, but we've made due with this for years, because managing 
text input would be complicated to implement.

Now note that the above method also works to search the help screen: get 
out of help, type in your search string, whether at a prompt or in a 
document or wherever, cut it, go into help and search for it. Clumsy, but 
much much better than nothing.