Support SystemD `Type=notify` #899

Job	Run time
check	4m 38s
matrix	37s
test-client-check	2m 26s
test-client-matrix	48s
test-client-clippy	2m 40s
Build test client (x86_64-unknown-linux-musl)	3m 52s
Build test client (i686-unknown-linux-musl)	3m 28s
Build test client (armv7-unknown-linux-musleabihf)	3m 40s
Build test client (aarch64-unknown-linux-musl)	3m 24s
Build test client (x86_64-unknown-freebsd)	3m 38s
Build test client (.exe, x86_64-pc-windows-gnu)	3m 37s
build (x86_64-unknown-linux-musl)	6m 23s
Check MSRV	4m 1s
clippy	3m 57s
build (i686-unknown-linux-musl)	6m 46s
tests	7m 31s
build (armv7-unknown-linux-musleabihf)	6m 46s
build (aarch64-unknown-linux-musl)	6m 51s
build (x86_64-unknown-freebsd)	6m 39s
Package nightly release	14m 34s
	1h 36m 16s

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