All our diagnostic macros required a struct notcurses from
which to extract the loglevel. Eliminate this parameter,
and update all caller sites. Add a library-wide (but not
exported) loglevel variable, and set it from both
notcurses_core_init() and ncdirect_core_init(). Add two
new direct mode flags, NCDIRECT_OPTION_VERBOSE and
NCDIRECT_OPTION_VERY_VERBOSE, mapping them to
NCLOGLEVEL_WARNING and NCLOGLEVEL_INFO respectively.
Closes#1690.
Add `notcurses_check_pixel_support()` and
`ncdirect_check_pixel_support()` per #1367. Removes
NCOPTION_VERIFY_SIXEL, again per #1367. Adds
`free_terminfo_cache()`, and calls it from both
`notcurses_stop_minimal()` and `ncdirect_stop()`.
Update all documentation. Closes#1371 and #1367.
Eliminate NCDIRECT_OPTION_NO_READLINE (it was only introduced in
Notcurses 2.1.6). Add a new function, ncdirect_readline(). Upon
first call to this function, initialize libreadline. Destroy
libreadline in ncdirect_stop() iff we initialized it. Add
hilodirect guessing game PoC from #1325. Rename
notcurses_directmode.3 to notcurses_direct.3. Closes#1326.