The while loop in xstrlcpy() breaks when -O3 is used.

1. The debug file is located at /tmp/nnn_debug.
2. Debug mode is completely disabled by default. The debug binary `nnndbg` can
be built by running `make debug`.

With the features the readline library provides it would be too naive to ignore.
We break out of the curses mode into the prompt and get back in.
This change introduces dependency on libncurses.

macOS 10.12.4 and many earlier versions have libncurses 5.4 with wide character
support, but not libncursesw.

Update README on dependencies

This is consistent with the fact that custom LDFLAGS are already allowed, and
for once, it makes warning options easier to control.

Also use automatic variables $@ and $^ consistently.

1. Have a faster copy of strlcpy()
2. Remove redundant legacy files
3. Fix an issue with NULL check on array name

Diff by Ypnose

The magic.mgc file gets created in the local directory, opening
the global file throws unwanted messages. So we are using the o/p
of the file command to determine if the file is a plain text file.

Use mpv, fmedia, zathura.
Invoke libmagic to identify text files and open with vim.
Use xdg-open for unhandled mimes.