analyze: various cleanups

Update systemd-analyze to follow the coding style of the other tools
more closely. Also, update the CODING_STYLE to document this for future
additions.

Changes:

- Always use usec_t for time units, so that we always use the same types
  everywhere, and format times the same way as everywhere else.

- Add "static" to global variables

- Make sure we can always distuingish OOM and other errors: ensure we
  always return useful error codes from all functions.

- Always free unit_times array