fix select exception fdset clearing

[russellromney]

Summary

Fix select(2) fdset handling when an exception event does not occur.

DoSelect() correctly clears readfds and writefds entries when their requested events are absent, but the exception-fdset branch accidentally cleared writefds instead of exceptfds:

w[i] &= !m;

This leaves the file descriptor incorrectly set in exceptfds.

Regression Test

Adds test/c/select_except_connect.c and includes it in make clean, a minimal loopback TCP regression:

1. start a local TCP listener
2. create a nonblocking client socket
3. call connect()
4. call select() with read/write/exception fdsets
5. assert the client socket is in writefds, not exceptfds, and SO_ERROR == 0

Validation

Local/native control:

select rc=1 read=0 write=1 except=0 so_error=0

Quark red/green on a DO droplet with only qkernel swapped:

Unpatched qkernel:

client socket was not in writefds
select rc=1 read=0 write=0 except=1 so_error=0

Patched qkernel:

select rc=1 read=0 write=1 except=0 so_error=0

runc control:

select rc=1 read=0 write=1 except=0 so_error=0