Allow proper tilde expansion overrides (#225)

Until now, when performing any tilde expansion like ~/foo or
~user/foo, ksh added a placeholder built-in command called
'.sh.tilde', ostensibly with the intention to allow users to
override it with a shell function or custom builtin. The multishell
ksh93 repo <https://github.com/multishell/ksh93/> shows this was
added sometime between 2002-06-28 and 2004-02-29. However, it has
never worked and crashed the shell.

This commit replaces that with something that works. Specific tilde
expansions can now be overridden using .set or .get discipline
functions associated with the .sh.tilde variable (see manual,
Discipline Functions).

For example, you can use either of:

.sh.tilde.set()
{
        case ${.sh.value} in
        '~tmp') .sh.value=${XDG_RUNTIME_DIR:-${TMPDIR:-/tmp}} ;;
        '~doc') .sh.value=~/Documents ;;
        '~ksh') .sh.value=/usr/local/src/ksh93/ksh ;;
        esac
}

.sh.tilde.get()
{
        case ${.sh.tilde} in
        '~tmp') .sh.value=${XDG_RUNTIME_DIR:-${TMPDIR:-/tmp}} ;;
        '~doc') .sh.value=~/Documents ;;
        '~ksh') .sh.value=/usr/local/src/ksh93/ksh ;;
        esac
}

src/cmd/ksh93/include/variables.h,
src/cmd/ksh93/data/variables.c:
- Add SH_TILDENOD for a new ${.sh.tilde} predefined variable.
  It is initially unset.

src/cmd/ksh93/sh/macro.c:
- sh_btilde(): Removed.
- tilde_expand2(): Rewritten. I started out with the tiny version
  of this function from the 2002-06-28 version of ksh. It uses the
  stack instead of sfio, which is more efficient. A bugfix for
  $HOME == '/' was retrofitted so that ~/foo does not become
  //foo instead of /foo. The rest is entirely new code.
     To implement the override functionality, it now checks if
  ${.sh.tilde} has any discipline function associated with it.
  If it does, it assigns the tilde expression to ${.sh.tilde} using
  nv_putval(), triggering the .set discipline, and then reads it
  back using nv_getval(), triggering the .get discipline. The
  resulting value is used if it is nonempty and does not still
  start with a tilde.

src/cmd/ksh93/bltins/typeset.c,
src/cmd/ksh93/tests/builtins.sh:
- Since ksh no longer adds a dummy '.sh.tilde' builtin, remove the
  ad-hoc hack that suppressed it from the output of 'builtin'.

src/cmd/ksh93/tests/tilde.sh:
- Add tests verifying everything I can think of, as well as tests
  for bugs found and fixed during this rewrite.

src/cmd/ksh93/tests/pty.sh:
- Add test verifying that the .sh.tilde.set() discipline does not
  modify the exit status value ($?) when performing tilde expansion
  as part of tab completion.

src/cmd/ksh93/sh.1:
- Instead of "tilde substitution", call the basic mechanism "tilde
  expansion", which is the term used everywhere else (including the
  1995 Bolsky/Korn ksh book).
- Document the new override feature.

Resolves: https://github.com/ksh93/ksh/issues/217

NEWS

@@ -5,6 +5,11 @@ Any uppercase BUG_* names are modernish shell bug IDs.
 
 2021-03-16:
 
+- Tilde expansion can now be extended or modified by defining a .sh.tilde.get
+  or .sh.tilde.set discipline function. This replaces a 2004 undocumented
+  attempt to add this functionality via a .sh.tilde built-in, which never
+  worked and crashed the shell. See the manual for details on the new method.
+
 - Fixed a bug in interactive shells: if a variable used by the shell called
   a discipline function (such as PS1.get() or COLUMNS.set()), the value of $?
   was set to the exit status of the discipline function instead of the last