zcommon: pre-iterate over sysfs instead of statting every feature

zcommon: pre-iterate over sysfs instead of statting every feature

If sufficient memory (<2K, realistically) is available, libzfs_init()
can be significantly shorted by iterating over the correct sysfs
directory before registrations, we can turn 168 stats into 15/18
syscalls (3 opens (6 if built in), 3 fstats, 6 getdentses, and 3
closes), a tenfoldish reduction; this is probably a bit faster, too.

The list is always optional, and registration functions (and one-off
users) can simply pass NULL, which will fall back to the previous
mechanism

Also, don't allocate in zfs_mod_supported_impl, and use use access()
instead of stat(), since existence is really what we care about

Also, fix pre-prop-checking compat in fallback for built-in ZFS

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #12089