autogen.sh, ngindent, platformtest.sh: Fix warnings of "shellcheck"

- SC2006: Use $(..) instead of legacy `..`.
- SC2015: Note that A && B || C is not if-then-else. C may run
          when A is true.
- SC2086: Double quote to prevent globbing and word splitting.
- SC2166: Prefer [ p ] && [ q ] as [ p -a q ] is not well defined.

contrib/ngindent

@@ -3,15 +3,15 @@
 INDENTARGS="-kr -i8 -ts8 -l80 -c3 -cd41 -ss -ncs -psl"
 
 # check if indent(1) is available
-type indent >/dev/null 2>&1 && INDENT="indent"
-type gindent >/dev/null 2>&1 && INDENT="gindent"
-type gnuindent >/dev/null 2>&1 && INDENT="gnuindent"
+command -v indent >/dev/null 2>&1 && INDENT="indent"
+command -v gindent >/dev/null 2>&1 && INDENT="gindent"
+command -v gnuindent >/dev/null 2>&1 && INDENT="gnuindent"
 
 if [ -z "$INDENT" ]; then
 	echo "Error: GNU \"indent\" not found!"
 	exit 1
 fi
 
-$INDENT -v $INDENTARGS $*
+$INDENT -v $INDENTARGS "$@"
 
 # -eof-