Ignore SIGINT and SIGQUIT for Pre-Install hooks

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