Fix minor shellcheck 0.7.2 warnings

The first warning of a misspelling is a false positive, so we annotate
the script accordingly. As for the x-prefix warnings update the check
to use the conventional '[ -z <string> ]' syntax.

all-syslog.sh:46:47: warning: Possible misspelling: ZEVENT_ZIO_OBJECT

may not be assigned, but ZEVENT_ZIO_OBJSET is. [SC2153]

make_gitrev.sh:53:6: note: Avoid x-prefix in comparisons as it no

longer serves a purpose [SC2268]

man-dates.sh:10:7: note: Avoid x-prefix in comparisons as it no

    longer serves a purpose [SC2268]

Reviewed-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #12208