Make devel/ncurses use terminfo-db,

Following up on the changes that happened in base, remove the terminfo-db
from the package and do not make it an explicit dependency: because
it fallbacks on termcap, it can work without terminfo-db

Add a message to tell the users about the terminfo-db so depending on their
preference they can chose to either stay with termcap or install terminfo-db

Reviewed by: swills (maintainer)
Approved by: swills (maintainer)
Differential Revision: https://reviews.freebsd.org/D29322