Fix shellcheck v0.4.6 warnings

Resolve new warnings reported after upgrading to shellcheck
version 0.4.6.  This patch contains no functional changes.

* egrep is non-standard and deprecated. Use grep -E instead. [SC2196]
* Check exit code directly with e.g. 'if mycmd;', not indirectly
  with $?.  [SC2181]  Suppressed.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #7040

scripts/zimport.sh

@@ -566,6 +566,7 @@ for TAG in $POOL_TAGS; do
 
 		$ZPOOL_CMD import -N -d "$POOL_DIR_COPY" \
 		   "$POOL_NAME" &>/dev/null
+		# shellcheck disable=SC2181
 		if [ $? -ne 0 ]; then
 			fail_nonewline
 			ERROR=1