filesystem-notify-evt: change inotify() implementation

Create a single inotify() connection per place, which should reduce
the latency of operations on filesystem change events and make
them generally scale better on Linux.

Internally, add a filesystem-never-changes mode, which could be useful
for platforms with fixed filesystems, but it's also for experiments in
checking the cost of filesystem change events.
add 'fs-change mode to `system-type'

Report whether `filesyste-change-evt' is supported, works
at the granularity of files, etc.
use `filesystem-change-evt' for collection paths only if low-latency

Specifically, this change disables the use of inotify() for detecting
changes to collection-links files, because the latency is too large
on some (many?) Linux configurations. (The fallback is to detect changes
to a links file by reading the whole content every time.)