Add a way to cleanly handle terminfo.db file used by devel/ncurses

This database contains compiled terminfo entries and is merged from
a large part which comes with ncurses themselves and compiled
.terminfo files installed by other ports (currently only rxvt-unicode
and jfbterm)

• Now the database is always kept up to date, regardless of port installation order (both ncurses installed before .terminfo files and the other way around work as expected)
• All affected ports now support clean deinstallation and do not produce stage-qa errors
• Affected ports are simplified, as they now only need to define USES=terminfo
• rxvt-unicode no longer pulls in ncurses: the dependency is not really needed for anything beside updating it's own database
• The patch contains Q/A test to check whether a port needs, or needs not USES=terminfo

Approved by: portmgr (mat, bapt)
Differential Revision: D3747