Issue 1063: change default guicursor behavior

[jgehrig]

Issue #1063: can't disable cursor styling

Currently, when no guicursor value is provided (styling is disabled), we use some default values: block, ver25, or hor25 based on mode.

The TUI does not behave the same way, it uses block mode for all cases when guicursor is empty.

This change removes the cursor defaults, and applies block mode to all cases where guicursor is empty.

[jgehrig]

I have some slight hesitancy on check-in for this fix, some users could complain of the exact opposite.

However, the TUI behaves as called out in the bug, so perhaps this will not be an issue?

[justinmk]

The TUI does not behave the same way, it uses block mode for all cases when guicursor is empty.

I think that also matches Vim. If the OS provides an API for "cursor style preferences", that would be ideal. Otherwise, yes, if the user explicitly unsets 'guicursor' they are asking for "do as little as possible".