Fix line continuation within command substitutions

In command substitutions of the $(standard) and ${ shared state; }
form, backslash line continuation is broken.

Reproducer:

	echo $(
	echo one two\
	three
	)

Actual output (ksh93, all versions):

	one two\ three

Expected output (every other shell, POSIX spec):

	one twothree

src/cmd/ksh93/sh/lex.c: sh_lex(): case S_REG:
- Do not skip new-line joining if we're currently processing a
  command substitution of one of these forms (i.e., if the
  lp->lexd.dolparen level is > 0).

Background info/analysis:

comsub() is called from sh_lex() when S_PAR is the current state.
In src/cmd/ksh93/data/lexstates.c, we see that S_PAR is reached in
the ST_DOL state table at index 40. Decimal 40 is ( in ASCII. So,
the previous skipping of characters was done according to the
ST_DOL state table, and the character that stopped it was (. This
means we have $(.

Alternatively, comsub() may be called from sh_lex() by jumping to
the do_comsub label. In brief, that is the case when we have ${.

Regardless of which it is from the two, comsub() is now called from
sh_lex(). In comsub(), lp->lexd.dolparen is incremented at the
beginning and decremented at the end. Between them, we see that
sh_lex() is called. So, lp->lexd.dolparen in sh_lex() indicates the
depth of nesting $( or ${ statements we're in. Thus, it is also the
number of comsub() invocations seen in a backtrace taken in
sh_lex().

The codepath for `...` is different (and never had this bug).

Co-authored by: Martijn Dekker <martijn@inlv.org>
Resolves: https://github.com/ksh93/ksh/issues/367

src/cmd/ksh93/sh/lex.c

@@ -671,8 +671,8 @@ int sh_lex(Lex_t* lp)
 				if(mode==ST_BEGIN)
 				{
 				do_reg:
-					/* skip new-line joining */
-					if(c=='\\' && fcpeek(0)=='\n')
+					/* skip new-line joining if not called from comsub() */
+					if(c=='\\' && fcpeek(0)=='\n' && !lp->lexd.dolparen)
 					{
 						sh.inlineno++;
 						fcseek(1);