etc/systemd/zfs-mount-generator: rewrite in C

etc/systemd/zfs-mount-generator: rewrite in C

A plain rewrite of the shell version, and generates identical
units, save for replacing some empty lines with nothing, having fewer
meaningless spaces in After=s and different spacing in the lock scripts,
for a clean git diff -w

This is a gain of anywhere from 0m0.336s vs 0m0.022s (15.27x)
to 0m0.202s vs 0m0.006s (33.67x), depending on the hardware,
a.k.a. from "absolutely unusable" to "perfectly fine"

This also properly deals with canmount=noauto units across multiple
pools

See PR for detailed timings (of an early version) and diffs

Reviewed-by: Antonio Russo <aerusso@aerusso.net>
Reviewed-by: Richard Laager <rlaager@wiktel.com>
Reviewed-by: InsanePrawn <insane.prawny@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Issue #11915
Closes #11917