Filter language improvements: mod()/random() and --no-filters (#714, #1232)

[eaescob]

Summary

This PR bundles two small filter-related improvements.

#714 — mod() and random() chance functions

• mod(offset, divisor) returns true when (field_value % divisor) != 0, e.g.:

  if (mod(tcp.seq, 2)) { drop(); kill(); }
  

• random() returns true 50% of the time.
• random(percent) returns true percent% of the time (0-100).
• The existing random(offset, start, length) data randomizer is preserved.

#1232 — --no-filters command-line flag

• New long option --no-filters loads filter files passed with -F/--filter but keeps them disabled at startup.
• Users can then enable them later through the interactive UI.
• The existing :0 / :1 filename suffix still overrides the per-file enabled state.

Test plan

• cmake --build build -j passes.
• etterfilter compiles and -t disassembles mod()/random() examples.
• ettercap -h shows the new --no-filters option.

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