Implement --enable-debuginfo to force debuginfo injection

Inspection of a Ubuntu 14.04 x64 system revealed that the config file
used to build the kernel image differs from the config file used to
build kernel modules by the presence of CONFIG_DEBUG_INFO=y:

root@zfs-dev:~# grep -E 'CONFIG_.*=' /boot/config-$(uname -r) | sort > /tmp/d1
root@zfs-dev:~# grep -E 'CONFIG_.*=' /usr/src/linux-headers-$(uname -r)/include/config/auto.conf | sort > /tmp/d2
root@zfs-dev:~# diff /tmp/d1 /tmp/d2
1017d1016
< CONFIG_DEBUG_INFO=y

This in itself is insufficient to show that the kernel is built with
debuginfo, but a cursory analysis of the debuginfo provided and the size
of the kernel strongly suggests that it was built with
CONFIG_DEBUG_INFO=y while the modules were not. Installing
linux-image-$(uname -r)-dbgsym had no obvious effect on the debuginfo
provided by either the modules or the kernel.

The consequence is that issue reports from distributions such as Ubuntu
and its derivatives build kernel modules without debuginfo contain
nonsensical backtraces. It is therefore desireable to force generation
of debuginfo, so we implement --enable-debuginfo. Since the build system
can build both userspace components and kernel modules, the generic
--enable-debuginfo option will force debuginfo for both. However, it
also supports --enable-debuginfo=kernel and --enable-debuginfo=user for
finer grained control.

Enabling debuginfo for the kernel modules works by injecting
CONFIG_DEBUG_INFO=y into the make environment. This is enables
generation of debuginfo by the kernel build systems on all Linux
kernels, but the build environment is slightly different int hat
CONFIG_DEBUG_INFO has not been in the CPP. Adding -DCONFIG_DEBUG_INFO
would fix that, but it would also cause build failures on kernels where
CONFIG_DEBUG_INFO=y is already set. That would complicate its use in
DKMS environments that support a range of kernels and is therefore
undesireable. We could write a compatibility shim to enable
CONFIG_DEBUG_INFO only when it is explicitly disabled, but we forgo
doing that because it is unnecessary. Nothing in ZoL or the kernel uses
CONFIG_DEBUG_INFO in the CPP at this time and that is unlikely to
change.

Enabling debuginfo for the userspace components is done by injecting -g
into CPPFLAGS. This is not necessary because the build system honors the
environment's CPPFLAGS by appending them to the actual CPPFLAGS used,
but it is supported for consistency.

Signed-off-by: Richard Yao <richard.yao@clusterhq.com>