linux-user: add signalfd/signalfd4 syscalls

This patch introduces a system very similar to the one used in the kernel
to attach specific functions to a given file descriptor.

In this case, we attach a specific "host_to_target()" translator to the fd
returned by signalfd() to be able to byte-swap the signalfd_siginfo
structure provided by read().

This patch allows to execute the example program given by
man signalfd(2):

 #include <sys/signalfd.h>
 #include <signal.h>
 #include <unistd.h>
 #include <stdlib.h>
 #include <stdio.h>

 #define handle_error(msg) \
     do { perror(msg); exit(EXIT_FAILURE); } while (0)

 int
 main(int argc, char *argv[])
 {
     sigset_t mask;
     int sfd;
     struct signalfd_siginfo fdsi;
     ssize_t s;

     sigemptyset(&mask);
     sigaddset(&mask, SIGINT);
     sigaddset(&mask, SIGQUIT);

     /* Block signals so that they aren't handled
        according to their default dispositions */

     if (sigprocmask(SIG_BLOCK, &mask, NULL) == -1)
         handle_error("sigprocmask");

     sfd = signalfd(-1, &mask, 0);
     if (sfd == -1)
         handle_error("signalfd");

     for (;;) {
         s = read(sfd, &fdsi, sizeof(struct signalfd_siginfo));
         if (s != sizeof(struct signalfd_siginfo))
             handle_error("read");

         if (fdsi.ssi_signo == SIGINT) {
             printf("Got SIGINT\n");
         } else if (fdsi.ssi_signo == SIGQUIT) {
             printf("Got SIGQUIT\n");
             exit(EXIT_SUCCESS);
         } else {
             printf("Read unexpected signal\n");
         }
     }
 }

 $ ./signalfd_demo
 ^CGot SIGINT
 ^CGot SIGINT
 ^\Got SIGQUIT

Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
