Ignore SIGINT and SIGQUIT for Pre-Install hooks

Instead of erroring out when receiving a SIGINT, let the
child deal with it - we'll error out anyway if the child
exits with an error or due to the signal. Also ignore
SIGQUIT, as system() ignores it.

This basically fixes Bug #832593, but: we are running
the hooks via sh -c. Some shells exit with a signal
error even if the command they are executing catches
the signal and exits successfully. So far, this has
been noticed on dash, which unfortunately, is our
default shell.

Example:
$ cat trap.sh
trap 'echo int' INT; sleep 10; exit 0
$ if dash -c ./trap.sh; then echo OK: $?; else echo FAIL: $?; fi
^Cint
FAIL: 130
$ if mksh -c ./trap.sh; then echo OK: $?; else echo FAIL: $?; fi
^Cint
OK: 0
$ if bash -c ./trap.sh; then echo OK: $?; else echo FAIL: $?; fi
^Cint
OK: 0

apt-pkg/deb/dpkgpm.cc

@@ -417,7 +417,6 @@ bool pkgDPkgPM::SendPkgsInfo(FILE * const F, unsigned int const &Version)
 bool pkgDPkgPM::RunScriptsWithPkgs(const char *Cnf)
 {
    bool result = true;
-   static bool interrupted = false;
 
    Configuration::Item const *Opts = _config->Tree(Cnf);
    if (Opts == 0 || Opts->Child == 0)
@@ -425,9 +424,8 @@ bool pkgDPkgPM::RunScriptsWithPkgs(const char *Cnf)
    Opts = Opts->Child;
 
    sighandler_t old_sigpipe = signal(SIGPIPE, SIG_IGN);
-   sighandler_t old_sigint = signal(SIGINT, [](int signum){
-	 interrupted = true;
-   });
+   sighandler_t old_sigint = signal(SIGINT, SIG_IGN);
+   sighandler_t old_sigquit = signal(SIGQUIT, SIG_IGN);
 
    unsigned int Count = 1;
    for (; Opts != 0; Opts = Opts->Next, Count++)
@@ -528,9 +526,7 @@ bool pkgDPkgPM::RunScriptsWithPkgs(const char *Cnf)
    }
    signal(SIGINT, old_sigint);
    signal(SIGPIPE, old_sigpipe);
-
-   if (interrupted)
-      result = _error->Error("Interrupted");
+   signal(SIGQUIT, old_sigquit);
 
    return result;
 }