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/sh.1

@@ -1664,6 +1664,16 @@ discipline function is invoked.
 .B .sh.subshell
 The current depth for subshells and command substitution.
 .TP
+.B .sh.pid
+Set to the process ID of the current shell.
+This is distinct from
+.B $$
+as in forked subshells this is set to the process ID of the
+subshell instead of the parent shell's process ID.
+In virtual subshells
+.B .sh.pid
+retains its previous value.
+.TP
 .B .sh.value
 Set to the value of the variable at the time that the
 .B set