Minimize included headers in implementation files and properly include headers in header files.

Replace one ncurses.h by stdio.h in input.c
Rename 'dcf77_mainloop' to just 'mainloop'
Sort include headers.
Formatting police, no functional changes.
Rename the parameter 'bit' of is_space_bit() to 'bitppos'.
Provide Javadoc-like descriptions for the public functions.
Install the header files in DESTDIR/PREFIX/include/dcf77pi
Allow 'make lint' to proceeds (chokes on ncurses).
Fix various warnings from 'make lint'

- The first parameter of mainloop() was unused, so remove it.
- The variable 'bitinf' in main() of dcf77pi.c was unused, so remove it.
- Make tmp3 signed because century_offset() can return -1 if no suitable century is found.
- Remove unused variable 'old_acc_minlen'
- Initialize 'bitpos' to 0.
- Use proper prototypes.