MFC 226145,226147,226148,226150,226151,226153,226157,226158,226164,226246,
226262,226329,226344,226608:
Merge most "infrastructure" changes to kdump to make other MFC's to add
new decodings, etc. easier:
- Some of the #defines or enums for which we auto-generate naming
  functions may be wider than int, so use intmax_t throughout.
- Auto-generate kdump_subr.h.
- Use a switch statement instead of a giant if-else.
- Remove a lot of (void) casts.
- Bring ioctlname() in line with all the other *name() functions, which
  actually print the name (or the numeric value, if they can't figure out
  the correct name) instead of just returning a pointer to it.  Also, since
  ioctl numbers are not and probably never will be unique, drop support for
  using a switch statement instead of an if/else chain.
- Mostly WARNS=6 clean.
- Update mkioctls to still work with both kdump and truss.