Problem:    Cannot use :unlet for an environment variable.
Solution:   Make it work.  Use unsetenv() if available. (Ken Takata,
            closes #2855)

Problem:    File permissions may end up wrong when writing.
Solution:   Use fchmod() instead of chmod() when possible.  Don't truncate
            until we know we can change the file.

Problem:    Cannot automatically get a server name in a terminal.
Solution:   Add the --enable-autoservername flag to configure. (Cimbali,
            closes #2317)

Problem:    Mac features are confusing.
Solution:   Make feature names more consistent, add "osxdarwin". Rename
            feature flags, cleanup Mac code. (Kazunobu Kuriyama, closes #2178)

Problem:    No terminal emulator support.  Cannot properly run commands in the
            GUI.  Cannot run a job interactively with an ssh connection.
Solution:   Very early implementation of the :terminal command.  Includes
            libvterm converted to ANSI C.  Many parts still missing.

Problem:    Since we only support GTK versions that have it, the ckeck for
            HAVE_GTK_MULTIHEAD is no longer needed.
Solution:   Remove HAVE_GTK_MULTIHEAD. (Kazunobu Kuriyama)

Problem:    Can't find executable name on Solaris and FreeBSD.
Solution:   Check for "/proc/self/path/a.out". (Danek Duvall) And for
            "/proc/curproc/file".

Problem:    v:progpath is not reliably set.
Solution:   Read /proc/self/exe if possible. (idea by Michal Grochmal)
            Also fixes missing #if.

Problem:    Getpgid is not supported on all systems.
Solution:   Add a configure check.

Problem:    Still checking if memcmp() exists while every system should have
            it now.
Solution:   Remove vim_memcmp().  (James McCoy, closes #1295)