Refactor --monotonic option and make it on by default

Also:
 * use the shorter "monontonic" variable name
 * make options global
 * remove unused option_index
 * add help text

BUG=none
TEST=mtplot --monotonic    # Enables monotonic timestamps
TEST=mtplot --monotonic=1  # Enables monotonic timestamps
TEST=mtplot --monotonic=0  # Disables monotonic timestamps
TEST=mtplot -m=0  # Disables monotonic timestamps
TEST=mtplot -m 1  # Enables monotonic timestamps
     # Note: "mtplot --monotonic 1" doesn't work

Change-Id: I3ab40c615cf5c763bf1971889d9e461e9a1df657
1 file changed