Set vi autoindent off by default

Will help prevent cascading indents when developing directly on router.

To test:
1. Copy a block of text with multiple indent levels (e.g., a few stanzas of /etc/config/network)
2. open a scratch file (e.g., `vi foo`) and paste. The text should appear properly formatted.
3. In the same file, in command mode (`<esc>`), type `:set ai`, then switch to insert and paste again. Each line of text should be indented one additional level.