sigpipe: factor out sigpipe_reset from easy.c

Commit 7d80ed64e43515 introduced some helpers to handle
sigpipe in easy.c. However, that fix was incomplete, and we
need to add more callers in other files. The first step is
making the helpers globally accessible.

Since the functions are small and should generally end up
inlined anyway, we simply define them in the header as
static functions.

Signed-off-by: Jeff King <peff@peff.net>
curl_multi_cleanup: ignore SIGPIPE

This is an extension to the fix in 7d80ed64e43515. We may
call Curl_disconnect() while cleaning up the multi handle,
which could lead to openssl sending packets, which could get
a SIGPIPE.

Signed-off-by: Jeff King <peff@peff.net>