create-diff-object: remove SHF_WRITE hack

We're no longer writing directly to the dynrela section, since the core
module has its own kpatch_dynrela data structures now.
fix undefined symbols for future loaded modules

When patching module A, if one of the new function's relas reference a
symbol in module B, we currently just leave it as a normal rela.  But if
module B hasn't been loaded yet, the patch module will fail to load due
to the rela's reference to an undefined symbol.

The fix is to convert these relas to dynrelas, which can be resolved
later in the module notifier when A is loaded.

Also added support for the R_X86_64_NONE relocation type, needed for
dynrelas which reference __fentry__.
remove unused kpatch_dynrela.objname field
Merge pull request #250 from jpoimboe/shf-write

create-diff-object: remove SHF_WRITE hack
fix review comments
Merge pull request #249 from jpoimboe/undefined-module-symbols

fix undefined symbols for future loaded modules
readme: several updates

- replace the old low-level video with the new high-level one, and place
  it with screenshot prominently at the top

- increase default ccache size

- remove obsolete NOTEs

- add blurb about patching modules and -t

- "trampoline" -> "handler"

- more details about ftrace, perf, tracepoints compatibility

- add recommended practice re: single cumulative module vs multiple
  independent modules
Merge pull request #251 from jpoimboe/readme-modules

readme: several updates
kpatch-build: fix getopt typo for --target
Merge pull request #252 from jpoimboe/dash-t-typo

kpatch-build: fix getopt typo for --target
kmod/core: fix unloaded module 'vmlinux' error

Fix the following error:

  [  344.564905] kpatch: delaying patch of unloaded module 'vmlinux'
Merge pull request #254 from jpoimboe/vmlinux-unloaded-err

kmod/core: fix unloaded module 'vmlinux' error