gitignore *.o.d

When the core module compilation fails, a kmod/core/.core.o.d file
appears.
kpatch: reformat usage message

In prepraration for "kpatch load --replace", reformat the usage message
to give more room for the command descriptions.  Also move the
formatting logic to its own function.
kpatch replace

Allow the user to atomically replace all existing modules with a new
"kpatch replace" command.  This provides a safe way to do atomic
upgrades for cumulative patch module updates.
Merge pull request #160 from jpoimboe/kpatch-load-replace

support for "kpatch replace"