Add a new make 'mancheck' target which uses mandoc -Tlint to verify
manpage files: currently only zfs(8), zpool(8) zdb(8) and zgenhostid(8)
are supported.
Additionally fix some outstanding manpage formatting issues.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes#6646
90cdf2833d