MEDIUM: fd: add fd_poll_{recv,send} for use when explicit polling is required

The old EV_FD_SET() macro was confusing, as it would enable receipt but there
was no way to indicate that EAGAIN was received, hence the recently added
FD_WAIT_* flags. They're not enough as we're still facing a conflict between
EV_FD_* and FD_WAIT_*. So let's offer I/O functions what they need to explicitly
request polling.
diff --git a/src/ev_sepoll.c b/src/ev_sepoll.c
index 62ee115..506ab31 100644
--- a/src/ev_sepoll.c
+++ b/src/ev_sepoll.c
@@ -215,6 +215,26 @@
  * Don't worry about the strange constructs in __fd_set/__fd_clr, they are
  * designed like this in order to reduce the number of jumps (verified).
  */
+REGPRM2 static void __fd_wai(const int fd, int dir)
+{
+	unsigned int i;
+
+#if DEBUG_DEV
+	if (!fdtab[fd].owner) {
+		fprintf(stderr, "sepoll.fd_wai called on closed fd #%d.\n", fd);
+		ABORT_NOW();
+	}
+#endif
+	i = ((unsigned)fdtab[fd].spec.e >> dir) & FD_EV_MASK_DIR;
+
+	if (!(i & FD_EV_IN_SL)) {
+		if (i == FD_EV_WAIT)
+			return; /* already in desired state */
+		alloc_spec_entry(fd); /* need a spec entry */
+	}
+	fdtab[fd].spec.e ^= (i ^ (unsigned int)FD_EV_IN_PL) << dir;
+}
+
 REGPRM2 static void __fd_set(const int fd, int dir)
 {
 	unsigned int i;
@@ -592,6 +612,7 @@
 
 	p->is_set  = __fd_is_set;
 	p->set = __fd_set;
+	p->wai = __fd_wai;
 	p->clr = __fd_clr;
 	p->rem = __fd_rem;
 	p->clo = __fd_clo;