Add epoll_pwait2().

I've also added doc comments for everything in <sys/epoll.h>.

I've also broken up the old "smoke" test (which was taking 2s on my
riscv64 qemu) to keep the total runtime for all the tests down to 200ms.

Test: treehugger
Change-Id: Icd939af51886fdf21432653a07373c1a0f26e422
diff --git a/libc/libc.map.txt b/libc/libc.map.txt
index e90618d..75b3d86 100644
--- a/libc/libc.map.txt
+++ b/libc/libc.map.txt
@@ -1586,6 +1586,8 @@
 
 LIBC_V { # introduced=VanillaIceCream
   global:
+    epoll_pwait2;
+    epoll_pwait2_64;
     localtime_rz;
     mbsrtowcs_l;
     mktime_z;