Fix 'make checkbashisms` warnings

The awk command used by the checkbashisms target incorrectly
adds the escape character before the ! and # characters.  This
results in the following warnings because these characters do not
need to be escaped.

    awk: cmd. line:1: warning: regexp escape sequence
        `\!' is not a known regexp operator
    awk: cmd. line:1: warning: regexp escape sequence
        `\#' is not a known regexp operator

Remove the unneeded escape character before ! and #.

Valid escape sequences are:

    https://www.gnu.org/software/gawk/manual/html_node/Escape-Sequences.html

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #11902

Makefile.am

@@ -162,7 +162,7 @@ checkbashisms:
 				-o -name '90zfs' -prune \
 				-o -type f ! -name 'config*' \
 				! -name 'libtool' \
-			-exec sh -c 'awk "NR==1 && /\#\!.*bin\/sh.*/ {print FILENAME;}" "{}"' \;); \
+			-exec sh -c 'awk "NR==1 && /#!.*bin\/sh.*/ {print FILENAME;}" "{}"' \;); \
 	else \
 		echo "skipping checkbashisms because checkbashisms is not installed"; \
 	fi