linux-user: refactor do_socketcall()

Refactor do_socketcall() to do argument conversion/checking first,
according to a lookup table (which call has how many args) and
by calling the right function second with ready-to-go arguments.

This ensures that all arguments are handled as abi_long, according
to socketcall prototype, and simplifies argument handling alot too.

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
