add functions for removing and modifying file descriptors to poller_t #109

Job	Run time
tests (ubuntu-latest, 98, gcc-10, g++-10, ON, 4.3.4, cmake)	1m 5s
tests (ubuntu-latest, 11, gcc-10, g++-10, ON, 4.3.4, cmake)	1m 3s
tests (ubuntu-latest, 20, gcc-10, g++-10, ON, 4.3.4, cmake)	1m 17s
tests (ubuntu-18.04, 11, gcc-7, g++-7, OFF, 4.2.0, pkgconfig)	1s
tests (ubuntu-18.04, 11, gcc-4.8, g++-4.8, ON, 4.3.4, cmake, gcc-4.8 g++-4.8)	1s
tests (ubuntu-18.04, 11, gcc-5, g++-5, ON, 4.3.4, cmake, gcc-5 g++-5)	1s
tests (ubuntu-latest, 20, gcc-10, g++-10, OFF, 4.3.4, cmake)	1m 2s
tests (ubuntu-latest, 17, gcc-9, g++-9, ON, 4.3.4, cmake, -DCOVERAGE=ON, lcov)	1m 37s
tests (ubuntu-latest, 17, clang-12, clang++-12, ON, 4.3.4, cmake)	1m 1s
tests (macos-latest, 17, clang, clang++, OFF, 4.3.4, false, zeromq)	1m 2s
tests (windows-2019, 14, msbuild, msbuild, ON, 4.3.4, cmake, -Ax64)	2m 51s
tests (windows-latest, 20, msbuild, msbuild, ON, 4.3.4, cmake, -Ax64)	4m 4s
	15m 5s

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