Avoid stat()ing all mount points on startup

[tavianator]

stat() can be slow, possibly hanging on network filesystems. We can at least delay it until we need to know an fstype for the first time. There's some alternatives:

• /proc/self/mountinfo: but be careful, see https://savannah.gnu.org/bugs/?50859
• statfs(): not portable, need to parse f_type
• AT_STATX_DONT_SYNC