While at it:
- directly assign 'fd' in eater from install_filter()
- turn op_cmp into a description-style thing

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

As discussed with Alice -- 'reuse lint' passes now.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

Add:
  - ignore_args field for the filter.
  - use MAX_FILTER to define the filter size in the eater

Only tangentially related:

- make seitan C99 again, so that I can build cooker without warnings

- make Makefiles make use of the usual conventions about assigning
  directory paths in variables, drop numbers.h as requirement for
  cooker and make it convenient to run stand-alone Makefiles

- fix up nr_syscalls.sh to be POSIX, otherwise it will give syntax
  errors on my system

- define a single, common way to refer to offsets in gluten, and
  functions to use those offsets in a safe way. Immediates are gone:
  cooker will write any bit of "data" to the read-only section

- call const what has to be const

- define on-disk layout for gluten

- add OP_NR (to check syscall numbers), rename OP_COPY_ARGS to
  OP_LOAD (it loads _selected_ stuff from arguments)

As for cooker itself:

- drop ARG_ and arg_ prefixes from struct names, and similar

- add/rework functions to build OP_NR, OP_LOAD, OP_CMP, and to
  write constant data to gluten

- add parsing for "compound" arguments, but that's not completely
  hooked into evaluation for numeric arguments yet

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>