core: Use eventfd for alert pipes on Linux when possible

The primary win of switching to eventfd when possible is that it only
uses a single file descriptor while pipe() will use two. This means for
each bridge channel we're reducing the number of required file
descriptors by 1, and - if you're using timerfd - we also now have 1
less file descriptor per Asterisk channel.

The API is not ideal (passing int arrays), but this is the cleanest
approach I could come up with to maintain API/ABI.

I've also removed what I believe to be an erroneous code block that
checked the non-blocking flag on the pipe ends for each read. If the
file descriptor is 'losing' its non-blocking mode, it is because of a
bug somewhere else in our code.

In my testing I haven't seen any measurable difference in performance.

Change-Id: Iff0fb1573e7f7a187d5211ddc60aa8f3da3edb1d

configure.ac

@@ -1125,6 +1125,15 @@ if test "${ac_cv_have_variable_fdset}x" = "0x"; then
 		AC_DEFINE([CONFIGURE_RAN_AS_ROOT], 1, [Some configure tests will unexpectedly fail if configure is run by a non-root user.  These may be able to be tested at runtime.]))
 fi
 
+AC_MSG_CHECKING([if we have usable eventfd support])
+AC_RUN_IFELSE(
+  [AC_LANG_PROGRAM([#include <sys/eventfd.h>],
+      [return eventfd(0, EFD_NONBLOCK | EFD_SEMAPHORE) == -1;])],
+  AC_MSG_RESULT(yes)
+  AC_DEFINE([HAVE_EVENTFD], 1, [Define to 1 if your system supports eventfd and the EFD_NONBLOCK and EFD_SEMAPHORE flags.]),
+  AC_MSG_RESULT(no)
+)
+
 AST_GCC_ATTRIBUTE(pure)
 AST_GCC_ATTRIBUTE(malloc)
 AST_GCC_ATTRIBUTE(const)