setopt: split up setopt_long and setopt_cptr

setopt: split up setopt_long and setopt_cptr

- Split setopt_long() into multiple functions
  (setopt_long_bool/net/http/proxy/ssl/proto/misc) and dispatch them
  using a small function table

- Extract proxy string option handling into setopt_cptr_proxy() and call
  it early from setopt_cptr()

- Reorder options to simplify #ifdef blocks and reduce per-function size

- Updates code paths to return CURLE_UNKNOWN_OPTION from helper defaults
  so dispatching can continue cleanly

Closes #20280