When using format `PSX_AUDIO_XA_FORMAT_XACD`, `psx_audio_xa_encode_init_sector` calls `psx_cdrom_init_sector`,
which calls `psx_cdrom_init_xa_subheader`, which zeroes out the subheader. But when using
`PSX_AUDIO_XA_FORMAT_XA`, `psx_audio_xa_encode_init_sector` doesn't call either init function, which leaves
`coding` uninitialized. All of the other subheader fields are assigned directly, so they aren't affected, but
`coding` is only OR'd with flags, so it can become corrupted. This could be fixed just by changing the first
OR with an assignment, but this PR instead calls `psx_cdrom_init_xa_subheader` directly when the format is
`PSX_AUDIO_XA_FORMAT_XA`.
* libpsxav: fix EOF flag being misapplied in xa/xacd output
* psxavenc: refactor str2/str2cd output to match xa/xacd output cleanup
* psxavenc: fix EDC not being calculated in 2336-byte sector mode
