Refine headers and eliminate dead code

[jserv]

Header hygiene:

• Move internal headers (fd-table.h, lkl-wrap.h, seccomp{,-defs}.h, shadow-fd.h, procmem.h, net.h, web.h, syscall-nr.h) from include/kbox to src/ to reduce public API surface
• Delete dead util.h/util.c (zero callers)
• Make file-local functions static in path.c, lkl-wrap.c, web-server.c
• Add KBOX_FD_TABLE_CAPACITY macro, kbox_fd_table_count(), kbox_web_set_fd_used()
• Refactor kbox_host_nrs to X-macro (KBOX_HOST_NR_FIELDS) for single source of truth between struct definition and syscall_name_from_nr()

Dead code elimination:

• Delete dead static wrappers from lkl-wrap.c (kbox_ensure_dev_console, kbox_lkl_dup3, kbox_lkl_execve, kbox_lkl_exit, kbox_lkl_wait4, etc.)
• Delete dead process-event pipeline (kbox_web_record_process, kbox_event_push_process, KBOX_EVT_PROCESS, struct kbox_process_event)
• Delete dead KBOX_EVT_COUNTER_DELTA enum and handler
• Delete dead forward_sendmsg (marked attribute((unused)))
• Remove unused slow_sample_ms config field
• Consolidate duplicate ELF parsing (kbox_parse_elf_interp delegates to kbox_find_elf_interp_loc)

Change-Id: I49da244c7d3c9eefd6abeed616f55dda064c3bbb

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Summary by cubic

Reduces the public C header surface and strips dead code to simplify the build and maintenance. Also makes devtmpfs/devpts mount failures non‑fatal so image boot works on lean LKL builds.

• Refactors

  • Moved internal headers from include/kbox to src/ (e.g., fd-table.h, lkl-wrap.h, seccomp{,-defs}.h, shadow-fd.h, procmem.h, net.h, web.h, syscall-nr.h); updated Makefile and pre-commit to include -Isrc.
  • Marked file-local functions static in path.c, lkl-wrap.c, and web-server.c.
  • Added KBOX_FD_TABLE_CAPACITY, kbox_fd_table_count(), and kbox_web_set_fd_used().
  • Converted kbox_host_nrs to the KBOX_HOST_NR_FIELDS X-macro and integrated with syscall_name_from_nr().
  • Consolidated ELF PT_INTERP parsing into a single code path.
  • Treat devtmpfs/devpts mount failures as warnings; proc, sysfs, and tmpfs remain fatal.

• Dead Code Removal

  • Removed util.h/util.c and unused wrappers from lkl-wrap.c (e.g., kbox_ensure_dev_console, kbox_lkl_dup3, kbox_lkl_execve, kbox_lkl_exit, kbox_lkl_wait4, etc.).
  • Dropped the process-event pipeline and related enums/structs (KBOX_EVT_PROCESS, KBOX_EVT_COUNTER_DELTA, and handlers).
  • Deleted forward_sendmsg and the unused slow_sample_ms config.

^{Written for commit a2ba2d2. Summary will update on new commits.}