feat: Add support for passing arbitrary ping options (config & command line)

[kazubu]

Adds a mechanism to pass arbitrary ping options through two paths:

• Config file (per-target): specify per target via pingopt=
• Command line (all targets / per IP version): -o / --ping-options-v4 / --ping-options-v6

This makes it possible to flexibly pass ping options such as packet size (-s)
or any other options depending on the target or IP version.

Usage

Config file (per-target)

Since the config splits tokens by whitespace, options are given as
comma-separated tokens:

googleDNS-bigpkt   8.8.8.8   pingopt=-s,1000,-w,2

Command line

# Common to all targets
./deadman -o="-s 1472" deadman.conf

# Separate options for IPv4 / IPv6
./deadman --ping-options-v4="-s 1472" --ping-options-v6="-s 1452" deadman.conf

Because the value starts with -, use the = form on the command line.

Order of application

Options are concatenated onto the ping command in this order:

common (-o)  →  per IP version (--ping-options-v4 / -v6)  →  per-target (config pingopt=)

Changes

• Add a pingopt argument to Ping / PingTarget, and inject the options when
  building the ping command in Ping.async_send() (dispatching v4/v6 via Ping.ipversion)
• Add a pingopt= key to the config parser (gettargetlist), splitting the
  comma-separated value into tokens
• Add the -o/--ping-options, --ping-options-v4, and --ping-options-v6
  command-line arguments
• Document the usage with examples in deadman.conf

Notes

• These options apply to regular ICMP ping (including via SSH / netns / vrf).
  The tcp / snmp / routeros_api paths do not use the ping command, so
  they are out of scope.