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
