init: Add readahead built-in command

Inspired by ag/2659809/, this CL add readahead built-in command in init
to let files be prefetched into pagecache for faster reading.
Readahead happens in background but due to filesystem limitation it
might take small amount of time in it reading the filesystem metadata
needed to locate the requested blocks. So the command is executed in a
forked process to not block init execution.

Bug: 62413151
Test: boottime, dumpcache
Change-Id: I56c86e2ebc20efda4aa509e6efb736bd1d92baa5