soft: evict a non-regular lock file without reading it

[dxbjavid]

_read_lock_file reads the soft lock file straight after opening it, without first checking it is a regular file. A same-uid peer can plant a fifo at the lock path with a writer attached, so the non-blocking read raises EAGAIN, which stale detection's suppress() quietly swallows; the file is then never broken and acquire just wedges until timeout (an empty non-blocking fifo read also raises EAGAIN on FreeBSD even without a writer). This mirrors the guard added to the soft read/write marker in #588: fstat first, and when the path is not a regular file report it as a malformed lock with its mtime and inode so the usual age-based eviction clears it rather than reading from it.