net-p2p/mktorrent: increase max piece size and make tracker optional

Optimal piece count is ~1500. mktorrent allows up to 2^28 currently, thus
providing ability to create torrents for 375 GiB of data with optimal piece
count. Increase maximum piece size to 2^30 thus allowing torrents for 1.5
TiB of data (again, with optimal piece count).

mktorrent currently can't create torrents without announce URL. Since DHT
works perfectly well nowadays, this limitation doesn't make much sense.
Change to require tracker only when creating private torrent (i.e. DHT,
PEX, etc. disabled).

While here: define LICENSE (GPLv2+), make options actually work, remove
redundant THREADS_DESC, and clean up Makefile a bit.

PR: 210374
Submitted by: Anton Sayetsky
Approved by: maintainer timeout