use __socketcall to simplify socket()

use __socketcall to simplify socket()

commit 59324c8b0950ee94db846a50554183c845ede160 added __socketcall
analogous to __syscall, returning the negated error rather than
setting errno. use it to simplify the fallback path of socket(),
avoiding extern calls and access to errno.

Author: Rich Felker <dalias@aerifal.cx>
Date:   Tue Jul 30 17:51:16 2019 -0400

    make __socketcall analogous to __syscall, error-returning