BUG/MINOR: external-checks: do not unblock undesired signals

BUG/MINOR: external-checks: do not unblock undesired signals

The external checks code makes use of block_sigchld() and unblock_sigchld()
to ensure nobody modifies the signals list while they're being manipulated.
It happens that these functions clear the list of blocked signals, so they
can possibly have a side effect if other signals are blocked. For now no
other signal is blocked but it may very well change in the future so rather
correctly use SIG_BLOCK/SIG_UNBLOCK instead of touching unrelated signals.

This fix should be backported to 1.6 for correctness.