Generate progs.h from a bunch of files instead of internal knowledge

apps/progs.pl counted on the caller to provide the exact command
files.  The unified build doesn't have that knowledge, and the easier
and more flexible thing to do is to feed it all the apps/*.c files and
let it figure out the command names by looking inside (looking for
/int ([a-z0-9][a-z0-9_]*)_main\(int argc,/).

Also, add it to the generate command, since it's a versioned file.

Reviewed-by: Rich Salz <rsalz@openssl.org>