Problem: plugin/shada.lua does not respect the `+` flag of 'cpoptions'
when writing a *.shada buffer into a different file.
Solution: Check the presence of `+` in 'cpoptions' and whether the
buffer is written to a different file in BufWriteCmd for
*.shada before resetting the 'modified' option of the buffer.
This little defect appeared after plugin/shada.vim was rewritten into
Lua in #34725.
Problem:
Vimscript functions return number to signal ok/error. Lua doesn't
convert these to be falsey.
Solution:
Explicitly check if the return value is 0.