kdump: Improve decoding of various *at calls:

* Write AT_FDCWD where appropriate.
* Decode the remaining arguments of openat() etc like open() etc.
Virtualize carp(4) variables to have per vnet control.

Reviewed by:	ae, glebius
- Only set ARCH_FLAGS (TARGET/TARGET_ARCH) if specified, otherwise
  allow the toolchain to detect the correct values.

- Remove {SRC,DOC,PORT}REVISION variables, and use 'branch@rNNNNNN'
  as the {SRC,DOC,PORT}BRANCH variables.

- Only set default KERNEL_FLAGS and WORLD_FLAGS make(1) jobs if the
  number of CPUs is greater than 1.

Submitted by:	hrs
- Remove the defaults for TARGET/TARGET_ARCH.

- Note that WORLD_FLAGS and KERNEL_FLAGS set the number of
  make(1) jobs only on SMP-capable systems.

MFC after:	3 days
X-MFC-With:	r254224
X-MFC-To:	stable/9, releng/9.2