*: Partial string handling refactoring

Main points:

- Replace `char_u` with `char` in some cases.
- Remove `str[len] = NUL` hack in some cases when `str` may be considered
  `const`.

src/nvim/pos.h

@@ -2,6 +2,8 @@
 #define NVIM_POS_H
 
 typedef long linenr_T;         // line number type
+/// Format used to print values which have linenr_T type
+#define PRIdLINENR "ld"
 
 /// Column number type
 typedef int colnr_T;