libselinux: add selinux_unshare() to unshare the SELinux namespace #38

Job	Run time
build (gcc, 3.12, 3.3)	1m 45s
build (gcc, 3.12, 3.3, test-flags-override)	1m 38s
build (gcc, 3.12, 3.3, test-debug)	1m 57s
build (gcc, 3.12, 3.3, linker-bfd)	2m 2s
build (gcc, 3.12, 3.3, linker-gold)	1m 29s
build (gcc, 3.11, 3.3)	1m 32s
build (gcc, 3.10, 3.3)	1m 33s
build (gcc, 3.9, 3.3)	1m 41s
build (gcc, 3.8, 3.3)	1m 47s
build (gcc, 3.7, 3.3)	1m 59s
build (gcc, pypy3.7, 3.3)	2m 13s
build (gcc, 3.12, 3.2)	1m 37s
build (gcc, 3.12, 3.1)	1m 42s
build (gcc, 3.12, 3.0)	1m 35s
build (gcc, 3.12, 2.7)	1m 54s
build (gcc, 3.12, 2.6)	2m 1s
build (gcc, 3.12, 2.5)	1m 54s
build (clang, 3.12, 3.3)	1m 48s
build (clang, 3.12, 3.3, test-flags-override)	1m 38s
build (clang, 3.12, 3.3, test-debug)	1m 27s
build (clang, 3.11, 3.3)	1m 38s
build (clang, 3.10, 3.3)	1m 44s
build (clang, 3.9, 3.3)	1m 42s
build (clang, 3.8, 3.3)	2m 33s
build (clang, 3.7, 3.3)	1m 43s
build (clang, pypy3.7, 3.3)	2m 20s
build (clang, 3.12, 3.2)	1m 42s
build (clang, 3.12, 3.1)	1m 40s
build (clang, 3.12, 3.0)	1m 47s
build (clang, 3.12, 2.7)	1m 56s
build (clang, 3.12, 2.6)	2m 12s
build (clang, 3.12, 2.5)	1m 51s
build (gcc, 3.12, 3.3, sanitizers)	1m 55s
	59m 55s

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