refactor device interface and add cmds: `ptable`, `bridge`, `devices` #1391

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10 warnings

ignoring return value of 'fgets' declared with attribute 'warn_unused_result' [-Wunused-result]

ignoring return value of 'write' declared with attribute 'warn_unused_result' [-Wunused-result]

ignoring return value of 'pipe' declared with attribute 'warn_unused_result' [-Wunused-result]

ignoring return value of 'write' declared with attribute 'warn_unused_result' [-Wunused-result]

format not a string literal and no format arguments [-Wformat-security]

#warning "poweroff unsupported, using halt as fallback" [-Wcpp]

variable 'opts' set but not used [-Wunused-but-set-variable]

"CSIZE" is not defined, evaluates to 0 [-Wundef]