socketpair: add eventfd and use SOCK_NONBLOCK for socketpair()

Currently, we use `pipe` for `wakeup_create`, which requires ***two***
file descriptors. Furthermore, given its complexity inside, `pipe` is a
bit heavyweight for just a simple event wait/notify mechanism.

`eventfd` would be a more suitable solution for this kind of scenario,
kernel also advocates for developers to use `eventfd` instead of `pipe`
in some simple use cases:

    Applications can use an eventfd file descriptor instead of a pipe
    (see pipe(2) in all cases where a pipe is used simply to signal
    events. The kernel overhead of an eventfd file descriptor is much
    lower than that of a pipe, and only one file descriptor is required
    (versus the two required for a pipe).

This change adds the new backend of `eventfd` for `wakeup_create` and
uses it where available, eliminating the overhead of `pipe`. Also, it
optimizes the `wakeup_create` to eliminate the system calls that make
file descriptors non-blocking by moving the logic of setting
non-blocking flags on file descriptors to `socketpair.c` and using
`SOCK_NONBLOCK` for `socketpair(2)`, `EFD_NONBLOCK` for `eventfd(2)`.

Ref:
https://man7.org/linux/man-pages/man7/pipe.7.html
https://man7.org/linux/man-pages/man2/eventfd.2.html
https://man7.org/linux/man-pages/man2/socketpair.2.html
https://www.gnu.org/software/gnulib/manual/html_node/eventfd.html

Closes #13874

lib/curl_config.h.cmake

@@ -419,6 +419,9 @@
 /* Define to 1 if you have the `pipe' function. */
 #cmakedefine HAVE_PIPE 1
 
+/* Define to 1 if you have the `eventfd' function. */
+#cmakedefine HAVE_EVENTFD 1
+
 /* If you have a fine poll */
 #cmakedefine HAVE_POLL_FINE 1
 
@@ -539,6 +542,9 @@
 /* Define to 1 if you have the timeval struct. */
 #cmakedefine HAVE_STRUCT_TIMEVAL 1
 
+/* Define to 1 if you have the <sys/eventfd.h> header file. */
+#cmakedefine HAVE_SYS_EVENTFD_H 1
+
 /* Define to 1 if you have the <sys/filio.h> header file. */
 #cmakedefine HAVE_SYS_FILIO_H 1