Extract cursor.h from misc{1,2}.h and memline.h
Enable -Wconversion on cursor.c
Remove ml_ prefix from cursor.h functions

s/ml_get_curline/get_cursor_line_ptr
s/ml_get_cursor/get_cursor_pos_ptr
Add cursor.{c,h} to clint-files.txt
Remove code duplication in get_cursor_rel_lnum
Merge #674 'Extract cursor.h from misc{1,2}.h and memline.h'