chronyd: add `prefer` and `maxsources` options #21621

Job	Run time
Feeds Package Test Build / Test aarch64_generic	2m 45s
Feeds Package Test Build / Test arm_cortex-a15_neon-vfpv4	2m 47s
Feeds Package Test Build / Test arm_cortex-a9_vfpv3-d16	2m 45s
Feeds Package Test Build / Test i386_pentium-mmx	2m 45s
Feeds Package Test Build / Test mips_24kc	2m 48s
Feeds Package Test Build / Test mipsel_24kc	2m 41s
Feeds Package Test Build / Test powerpc_464fp	2m 43s
Feeds Package Test Build / Test powerpc_8548	2m 43s
Feeds Package Test Build / Test riscv64_riscv64	2m 40s
Feeds Package Test Build / Test x86_64	2m 41s
	27m 18s

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