net: permit pollster DelFD to return whether to call Wakeup

This is necessary for systems that use select as the pollster,
such as Solaris (supported by gccgo).  It corresponds to the
bool returned by AddFD.  In general it's not clearly defined
what happens when a descriptor used in a select is closed, and
different systems behave differently.  Waking up the select
will cause the right thing to happen: the closed descriptor
will be dropped from the next iteration.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/7303056
crypto/rc4: naïve ARM assembly implementation

On 800MHz Cortex-A8:
benchmark           old ns/op    new ns/op    delta
BenchmarkRC4_128         9395         2838  -69.79%
BenchmarkRC4_1K         74497        22120  -70.31%
BenchmarkRC4_8K        587243       171435  -70.81%

benchmark            old MB/s     new MB/s  speedup
BenchmarkRC4_128        13.62        45.09    3.31x
BenchmarkRC4_1K         13.75        46.29    3.37x
BenchmarkRC4_8K         13.79        47.22    3.42x

Result for "OpenSSL 1.0.1c 10 May 2012" from Debian/armhf sid:
type             16 bytes     64 bytes    256 bytes   1024 bytes   8192 bytes
rc4              39553.81k    46522.39k    49336.11k    50085.63k    50258.06k

R=golang-dev, agl, dave
CC=golang-dev
https://codereview.appspot.com/7310051
os/exec: LookPath on Unix shouldn't look in cwd when PATH is empty

R=golang-dev, dave, bradfitz
CC=golang-dev
https://codereview.appspot.com/7305053

Committer: Brad Fitzpatrick <bradfitz@golang.org>
net: skip TestMulticastListener on Solaris

R=golang-dev, minux.ma
CC=golang-dev
https://codereview.appspot.com/7308060