Problem: The option.c file is too big.
Solution: Split off the code dealing with strings. (Yegappan Lakshmanan,
closesvim/vim#4937)
dac1347b4d
Cherry-pick set_string_option_direct_in_win() from patch 8.1.1405.
Cherry-pick shift_line() comment change from patch 8.1.2096.
Move 'clipboard' default parsing to didset_string_options().
Reorder option flags to put Nvim-only flags at the end.