src/cursor.c: cursor names: use absence of "grab" to trigger fallback

src/cursor.c: cursor names: use absence of "grab" to trigger fallback

This is necessary because wlroots backported a patch containing
cursor name aliases to the 0.16 branch. That internal mapping
does not alias "grab" to "grabbing" because it is not technically
part of the cursor-names spec.

To keep the "grab" cursor working on systems with no cursor theme
available (or in an unusual location) change the trigger for our
internal cursor name aliasing to "grab".

See wlroots commit 1e345c74c3ca5d2575aa975d4d9af87cd0c7ba13
Alias existing cursor defaults to cursor-spec cursor names

for further details.