preload: fix sigmask block and restore race #1094

Job	Run time
OS (apt-docker.io/amd64/ubuntu:devel)	2m 3s
OS (apt-docker.io/amd64/ubuntu:latest)	1m 59s
OS (apt-docker.io/amd64/ubuntu:rolling)	1m 49s
OS (apt-docker.io/i386/debian:testing)	48s
OS (apt-docker.io/amd64/debian:testing)	43s
OS (dnf-registry.fedoraproject.org/fedora:latest)	2m 1s
OS (dnf-registry.fedoraproject.org/fedora:rawhide)	2m 5s
OS (alpine-docker.io/alpine)	34s
OS (alpine-docker.io/i386/alpine)	44s
OS ([email protected]/alpine)	33s
OS (nix-docker.io/nixos/nix)	1m 12s
OS (gentoo-)	6m 43s
OS (alpine@gudev-alpine, EXTRA_PACKAGES=libgudev-dev)	36s
	21m 50s

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