creddit: Move LinkScreen to its own file

This patch refactors the LinkScreen code by moving it from ./src/main.c
into the files ./src/screens/link_screen.c and
./src/include/screens/link_screen.h.
creddit: Move CommentScreen to its own file

This patch refactors the CommentScreen code by moving it from
./src/main.c into the files ./src/screens/comment_screen.c and
./src/include/screens/comment_screen.h.
creddit: Move _XOPEN_SOURCE_EXTENDED define to global.h

This patch moves the _XOPEN_SOURCE_EXTENDED define from the individual
files into global.h
creddit: Add 'RMDIR' macro to Makefile setup

This patch adds a 'RMDIR' macro to the Makefile setup. Originally 'rm
-fr' was relied on working, but on machines where RM may be changed out,
RM -fr won't work. This simplifies compiling on those machines.
creddit: Align Makefile filenames

This patch aligns the Filenames in the output from make so it's easier
to read.