Add help option to bootchart man page

Bootchart has a help option. For the sake of consistency, this patch
adds it to the man page.

Also, the TODO is updated. Bootcharts were added to the journal in
commit c4d58b0.
Dynamically allocate bootchart logs

Instead of storing bootchart sample data in arrays, this patch moves
storage to linked lists so that there is no more limit on samples.

This patch also fixes parsing of /proc/<pid>/smaps in kernels > 3.7.
bootchart: cleanup unused structs and globals