Allow unlinkat() in the seccomp2 sandbox

[nmathewson]

Closes ticket 33346.

[coveralls]

Pull Request Test Coverage Report for Build 8214

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1260 unchanged lines in 39 files lost coverage.
• Overall coverage decreased (-1.7%) to 59.194%

---

Files with Coverage Reduction	New Missed Lines	%
src/core/or/or.h	1	88.89%
src/feature/dirclient/dirclient.c	1	18.5%
src/feature/hs/hs_service.c	1	73.37%
src/feature/nodelist/nodelist.c	1	61.26%
src/lib/string/compat_ctype.h	1	88.89%
src/core/or/circuitlist.c	2	55.45%
src/core/or/policies.c	2	84.6%
src/feature/dircache/consdiffmgr.c	2	90.06%
src/feature/nodelist/microdesc.c	2	73.12%
src/feature/relay/router.c	2	35.2%
<!--	Total:	1260

Totals
Change from base Build 8056:	-1.7%
Covered Lines:	43299
Relevant Lines:	73148

---

💛 - Coveralls