kpatch-build: download correct source for the current kernel (for ubuntu)

As per issue #239.
kpatch-build: add error message if dpkg -x fails to extract source
kpatch-build: redirect output of wget, dpkg, and tar to log file, add error message if tar fails
Merge pull request #240 from flaming-toast/issue239

kpatch-build: download correct source for the current kernel (for ubuntu)
kpatch-build: ubuntu fix for vmlinux_3 error

kpatch load fails on Ubuntu with:

  kpatch: unable to find module 'vmlinux_3'

The root cause is that the vmlinux file on Ubuntu is named
vmlinux-3.13.0-24-generic instead of just vmlinux.

Let's just call it "vmlinux" in the objname field.
kpatch-build: add ccache symlinks to PATH

On Ubuntu, the ccache symlinks aren't automatically added to the PATH,
so update PATH accordingly.

On Fedora, the PATH is updated automatically when installing ccache.
_But_, it requires a new bash session to be created after installing
ccache before the new PATH takes effect.  So it's a good idea to fix it
for Fedora as well.
Merge pull request #241 from jpoimboe/ubuntu-fixes

Ubuntu fixes