Guard against nil in mu4e~guess-maildir (Fixes #472)

Add a guard so we don't error out if string-match evaluates to nil
Check for specific gmime-2.6 version (fixes #473)

Apparently, some gmime-2.5 versions match gmime-2.6 (pkgconfig) but
don't have the required API.

2.6.10 (2+ years old) is reported to work; test for this.
Some documentation fixes

(Thanks to Joe Corneli)
Bump version to 0.9.10
Rework getting database version (#469)

It seems we don't get the correct database version in some case with
some compilers, optimization flags. Suspecting some stale ->c_str().
Document that mu4e-maildir must not be a symlink ('fixes' #453)

mu doesn't support symlinks; document this. You can use hardlinks
instead of course.
Allow delete keybinding in console view-mode

Or, fix typo
Allow for gmime-2.6.7

See a86138a5. Apparently gmime 2.6.7 works fine as well.
Don't use g_clear_pointer yet, it's too new.

Do it the old way.