Add optional support for sending systemd readiness notifications #5008

Job	Run time
clang-format	1m 10s
build_test (LLVM 10 Release, Release, .#bpftrace-llvm10, tcpdrop.bt, biosnoop.bt)	10m 26s
build_test (LLVM 11 Release, Release, .#bpftrace-llvm11, tcpdrop.bt, biosnoop.bt)	10m 28s
build_test (LLVM 12 Release, Release, .#bpftrace-llvm12, tcpdrop.bt, biosnoop.bt)	12m 43s
build_test (LLVM 13 Release, Release, .#bpftrace-llvm13, tcpdrop.bt, biosnoop.bt)	10m 20s
build_test (LLVM 14 Release, Release, .#bpftrace-llvm14, tcpdrop.bt, biosnoop.bt)	10m 12s
build_test (LLVM 15 Release, Release, .#bpftrace-llvm15, tcpdrop.bt, biosnoop.bt)	10m 16s
build_test (LLVM 16 Release, Release, .#bpftrace-llvm16, tcpdrop.bt, biosnoop.bt)	10m 30s
build_test (LLVM 17 Release, Release, .#bpftrace-llvm17, tcpdrop.bt, biosnoop.bt)	10m 28s
build_test (LLVM 17 Debug, Debug, .#bpftrace-llvm17, tcpdrop.bt, biosnoop.bt)	12m 20s
build_test (LLVM 17 Clang Debug, Debug, .#bpftrace-llvm17, clang, clang++, tcpdrop.bt, biosnoop.bt)	10m 36s
build_test (Memleak test (LLVM 11 Debug), Debug, .#bpftrace-llvm11, 1, 0)	6m 42s
build_test (Memleak test (LLVM 11 Release), Release, .#bpftrace-llvm11, 1, 0)	5m 24s
irc	0s
	2h 1m 35s

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