Problem:    Crash when a callback deletes a window that is being used.
Solution:   Do not unload a buffer that is being displayed while redrawing the
            screen. Also avoid invoking callbacks while redrawing.
            (closes #2107)

Problem:    Too many #ifdefs.
Solution:   Graduate the +vreplace feature, it's not much code and quite a few
            #ifdefs.

Problem:    Build failure when disabling the session feature. (Pawel Slowik)
Solution:   Adjust #ifdef for vim_chdirfile().

Problem:    MS-Windows: term_start() does not set job_info() cmd.
Solution:   Share the code from job_start() to set jv_argv.

Problem:    Build failure on MS-Windows.
Solution:   Build job arguments for MS-Windows. Fix allocating job twice.

Problem:    The terminal API "drop" command doesn't support options.
Solution:   Implement the options.

Problem:    Reading spell file has no good EOF detection.
Solution:   Check for EOF at every character read for a length field.

Problem:    Too many #ifdefs.
Solution:   Graduate the +autocmd feature. Takes away 450 #ifdefs and
            increases code size of tiny Vim by only 40 Kbyte.

Problem:    Clearing a pointer takes two lines.
Solution:   Add VIM_CLEAR() and replace vim_clear(). (Hirohito Higashi,
            closes #2629)

Problem:    Memory leak in balloon_split().
Solution:   Free the balloon lines. Free the balloon when exiting.