pmcannotate: Add a '-m' mode option to control per-line annotations.

• "block" (the default mode) displays per-block percentages in front of each annotated line.

• "global" displays a global percentage in front of each line.

• "raw" displays the raw count of sample hits in front of each line.

Reviewed by: Pau Amma <pauamma@gundo.com>, mav
Sponsored by: University of Cambridge, Google, Inc.
Differential Revision: https://reviews.freebsd.org/D35401