indent(1): Optimize parser stack usage.

When special else-if processing is enabled (-ei), we can assume "else if"
and "if" to be equivalent for indentation purposes.
This reduction saves a lot of stack space in case of a long "if-else-if
... else-if" sequence;  with this change,
Postgres/src/bin/psql/tab-complete.c as of 9.6beta3
requires minimum of the stack length to be 31 instead of 444.

Submitted by:	 Piotr Sephaniak

usr.bin/indent/parse.c

@@ -94,7 +94,13 @@ parse(int tk) /* tk: the code for the construct scanned */
 
     case ifstmt:		/* scanned if (...) */
 	if (ps.p_stack[ps.tos] == elsehead && ps.else_if)	/* "else if ..." */
-	    ps.i_l_follow = ps.il[ps.tos];
+		/*
+		 * Note that the stack pointer here is decremented, effectively
+		 * reducing "else if" to "if". This saves a lot of stack space
+		 * in case of a long "if-else-if ... else-if" sequence.
+		 */
+		ps.i_l_follow = ps.il[ps.tos--];
+
 	/* the rest is the same as for dolit and forstmt */
     case dolit:		/* 'do' */
     case forstmt:		/* for (...) */