eloop: Add epoll option for better performance

eloop: Add epoll option for better performance

This patch adds epoll option for the eloop implementation. This can be
selected with the CONFIG_ELOOP_EPOLL=y build option.

[merit]
See Table1.

Table1. comparison table
+--------+--------+-----------+------------+-------------+
|        | add fd | remove fd | prepare fd | dispatch fd |
+--------+--------+-----------+------------+-------------+
| select | O(1)   | O(1)      | O(N)       | O(N)        |
+--------+--------+-----------+------------+-------------+
| poll   | O(1)   | O(1)      | O(N)       | O(N)        |
+--------+--------+-----------+------------+-------------+
| epoll  | O(1)   | O(1)      | 0          | O(M)        |
+--------+--------+-----------+------------+-------------+
"add fd" is addition of fd by eloop_sock_table_add_sock().
"remove fd" is removal of fd by eloop_sock_table_remove_sock().
"prepare fd" is preparation of fds before wait in eloop_run().
"dispatch fd" is dispatchment of fds by eloop_sock_table_dispatch().
"N" is all watching fds.
"M" is fds which could be dispatched after waiting.

As shown in Table1, epoll option has better performance on "prepare fd" column.
Because select/poll option requires setting fds before every select()/poll().
But epoll_wait() doesn't need it.

And epoll option has also better performance on "dispatch fd" column.
Because select/poll option needs to check all registered fds to find out
dispatchable fds. But epoll option doesn't require checking all registered fds.
Because epoll_wait() returns dispatchable fd set.

So epoll option is effective for GO/AP functionality.

[demerit]
The epoll option requires additional heap memory. In case of P2P GO, it is
about 8K bytes.

Signed-off-by: Masashi Honma <masashi.honma@gmail.com>