Add ${.sh.pid} as an alternative to $BASHPID (#109)

This variable is like Bash's $BASHPID, but in virtual subshells
it will retain its previous value as virtual subshells don't fork.
Both $BASHPID and ${.sh.pid} are different from $$ as the latter
is only set to the parent shell's process ID (i.e. it isn't set
to the process ID of the current subshell).

src/cmd/ksh93/include/defs.h:
- Add 'current_pid' for storing the current process ID at a valid
  memory address.
- Change 'ppid' from 'int32_t' to 'pid_t', as the return value from
  'getppid' is of the 'pid_t' data type.

src/cmd/ksh93/data/variables.c,
src/cmd/ksh93/include/variables.h,
src/cmd/ksh93/sh/init.c,
src/cmd/ksh93/sh/xec.c:
 - Add the ${.sh.pid} variable as an alternative to $BASHPID.
   The process ID is stored in a struct before ${.sh.pid} is set
   as environment variables are pointers that must point to a
   valid memory address. ${.sh.pid} is updated by the _sh_fork()
   function, which is called when ksh forks a new process with
   sh_fork() or sh_ntfork().

src/cmd/ksh93/tests/variables.sh:
- Add ${.sh.pid} to the list of special variables and add three
  regression tests for ${.sh.pid}.

src/cmd/ksh93/tests/subshell.sh:
- Update the PATH forking regression test to use ${.sh.pid} and
  remove the TODO note.

src/cmd/ksh93/include/defs.h

@@ -122,7 +122,8 @@ struct shared
 	gid_t		groupid;
 	gid_t		egroupid;
 	pid_t		pid;
-	int32_t		ppid;
+	pid_t		ppid;
+	pid_t		current_pid;  /* pid of the current ksh instance */
 	unsigned char	sigruntime[2];
 	Namval_t	*bltin_nodes;
 	Namval_t	*bltin_cmds;

src/cmd/ksh93/include/variables.h

@@ -90,6 +90,7 @@
 #define SH_STATS	(shgd->bltin_nodes+60)
 #define SH_MATHNOD	(shgd->bltin_nodes+61)
 #define SH_JOBPOOL	(shgd->bltin_nodes+62)
-#define SHLVL		(shgd->bltin_nodes+63)
+#define SH_PIDNOD	(shgd->bltin_nodes+63)
+#define SHLVL		(shgd->bltin_nodes+64)
 
 #endif /* SH_VALNOD */