Rewrite IN_SET()

This restores proper speed with asan builds with gcc 9.1.1.
Fixes #12997.

$ rpm -q gcc
gcc-9.1.1-2.fc31.x86_64

$ time ASAN_OPTIONS=strict_string_checks=1:detect_stack_use_after_return=1:check_initialization_order=1:strict_init_order=1 UBSAN_OPTIONS=print_stacktrace=1:print_summary=1:halt_on_error=1 build-rawhide-sanitize/test-conf-parser

(old) 86.99s user 20.22s system 361% cpu 29.635 total
(new)  3.05s user  0.29s system  99% cpu  3.377 total

Size is increased a bit:

$ size build/systemd
(old) 1683421	 246100	   1208	1930729	 1d75e9	build/systemd
(new) 1688237	 246100	   1208	1935545	 1d88b9	build/systemd

... but that's <0.1%, so we don't really care.