wg: More style cleanups and minor updates to if_wg.c

wg: More style cleanups and minor updates to if_wg.c

- Clean up header inclusions.
- Add 'WG_PKT_WITH_PADDING()' and 'WG_PKT_DATA_MINLEN' macros to help
  simplify some code.
- Remove unnecessary assignments that are already ensured by
  kmalloc(M_ZERO).
- Rename some variables for consistency.
- Use 'int' instead of 'u_int' for cpu, in consistent with 'ncpus'.
- Use 'memcmp()' instead of 'bmcp()' for consistent with other parts.
- Remove the KKASSERT() of lock assertion from 'wg_aip_remove()', as
  it's quite obvious and thus unnecessary.
- Various style adjustments and cleanups.