MINOR: signal: don't block SIGPROF by default

SIGPROF is blocked then restored to default settings, which sometimes
makes profiling harder. Let's not block it by default nor restore it,
it doesn't serve any purpose anyway.
OPTIM: epoll: make use of EPOLLRDHUP

epoll may report pending shutdowns using EPOLLRDHUP. Since this
flag is missing from a number of libcs despite being available
since kernel 2.6.17, let's define it ourselves.

Doing so saves one syscall by allow us to avoid the read()==0 when
the server closes with the respose.
OPTIM: splice: detect shutdowns and avoid splice() == 0

Since last commit introducing EPOLLRDHUP, the splicing code is able to
detect an incoming shutdown without calling splice() == 0. This avoids
one useless syscall.
OPTIM: splice: assume by default that splice is working correctly

Versions of splice between 2.6.25 and 2.6.27.12 were bogus and would return EAGAIN
on incoming shutdowns. On these versions, we have to call recv() after such a return
in order to find whether splice is OK or not. Since 2.6.27.13 we don't need to do
this anymore, saving one useless recv() call after each splice() returning EAGAIN,
and we can avoid this logic by defining ASSUME_SPLICE_WORKS.

Building with linux2628 automatically enables splice and the flag above since the
kernel is safe. People enabling splice for custom kernels will be able to disable
this logic by hand too.