[SR-7316] libdispatch: DISPATCH_SOURCE_TYPE_DATA_* does not work with the epoll backend (linux)

Issue Description:

The following C program will crash for me:

#include <dispatch/dispatch.h>

int main(int argc, char **argv, char **envp) {
{{ dispatch_queue_t main_queue = dispatch_get_main_queue();}}
{{ dispatch_source_t source = dispatch_source_create(DISPATCH_SOURCE_TYPE_DATA_ADD, 0, 0, main_queue);}}
{{ dispatch_resume(source);}}
{{ dispatch_main();}}
};

The issue is within _dispatch_muxnote_create of the epoll backend, this code hits the DISPATCH_INTERNAL_CRASH on line src/event/event_epoll.c:200.

Adding a case for DISPATCH_EVFILT_CUSTOM_ADD in that switch statement solves the crash, but I don't actually know what it is supposed to do if I add it. Limited testing (with actual event handlers) did not uncover any obvious bugs for me with no code in the case.

Installing a kqueue/kevent compatibility library and changing the build to use the kevent backend also works on my linux installation. I guess the epoll backend should be fixed, but in addition, I would like to propose a build time option to switch the backend on linux systems that have kqueue/kevent.

(DISPATCH_SOURCE_TYPE_DATA_OR has the same issue.)

Start with the reproducer and inspect _dispatch_muxnote_create in src/event/event_epoll.c, especially the DISPATCH_INTERNAL_CRASH at line 200 and the DISPATCH_EVFILT_CUSTOM_ADD switch case. Compare the epoll and kevent backends, then verify DISPATCH_SOURCE_TYPE_DATA_ADD and DISPATCH_SOURCE_TYPE_DATA_OR with actual event handlers. Done means these sources no longer crash on Linux with epoll.