top: Swich some uses of libc to rustix

[tonybajan]

Use rustix instead libc to provide getpriority, setpriorty and page size calls.

Part of #599

There's some remaining uses of uucore::libc after this change:

• types for calls to libsystemd (sd_booted, sd_get_sessions) + windows SystemProcessorPerformanceInformation
• calls to sched_getscheduler and sched_getparam that aren't implemented in rustix