Preprocessor flags are passed in CPPFLAGS.
Debian uses it to pass hardening options (-D_FORTIFY_SOURCE)
to the compiler.

- `nnn` is all about navigational convenience so the user doesn't have to type
- the binary size increases due to readline linkage
- alternative workflow: ^G, cd, `nnn`
- readline required using the prompt and the history would stay after quit

This fixes the build with the BSD install command by eschewing the GNU 
specific -t option, which is not portable.

* make Makefile more packaging friendly

* packagecore: run `strip` before installing

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