readahead: Add tool to analyze the contents of the pack file. v3.

This patch adds code to compile 'systemd-readahead-analyze' and install
it into $bindir.

Use this program to parse the contents of the readahead pack file, or
an arbitrary pack file and display which files are listed in it, and
how much of the files are requested to be readahead.

This code is not new - it's partially taken from sreadahead (formerly
maintained by Arjan van der Ven and me, and was originally written
by me), and adapted with the right bits to parse the systemd
readahead pack files, which are slightly different in format.

v2 adds a common READAHEAD_PACK_FILE_VERSION used in all the code
to provide a quick way to assure all these programs are always
synchronized. v3 fixes the integer math.