channel: Remove old epoll support and fixed max number of file descriptors.

This change removes the old epoll support which has not been used or
maintained in quite some time.

The fixed number of file descriptors on a channel has also been removed.
File descriptors are now contained in a growable vector. This can be
used like before by specifying a specific position to store a file
descriptor at or using a new API call, ast_channel_fd_add, which adds
a file descriptor to the channel and returns its position.

Tests have been added which cover the growing behavior of the vector
and the new API call.

ASTERISK-26885

Change-Id: I1a754b506c009b83dfdeeb08c2d2815db30ef928