Untangle lib/fsm.c and lib/cpio.c

Untangle lib/fsm.c and lib/cpio.c

Create cpio_t data type that holds the underlaying FD_t instance
Move padding and position handling and trailer generation into cpio.c
Use only one buffer in the fsm (merging the read and write buffer)
Replace the FSM_EAT, FSM_POS, FSM_PAD, FSM_DREAD, FSM_DWRITE states with cpio functions
Prepend "rpm" to the cpio function names