Reinstate 'r' and 'history' as preset aliases for interactive ksh

Following a community discussion, it became clear that 'r' is
particularly problematic as a regular builtin, as the name can and
does conflict with at least one legit external command by that
name. There was a consensus against removing it altogether and
letting users set the alias in their login scripts. However,
aliases are easier to bypass, remove or rename than builtins are.
My compromise is to reinstate 'r' as a preset alias on interactive
shells only, along with 'history', as was done in 17f81ebe before
they were converted to builtins in 03224ae3. So this reintroduces
the notion of predefined aliases to ksh 93u+m, but only for
interactive shells that are not initialised in POSIX mode.

src/cmd/ksh93/Makefile,
src/cmd/ksh93/Mamfile,
src/cmd/ksh93/include/shtable.h,
src/cmd/ksh93/data/aliases.c:
- Restore aliases.c containing shtab_aliases[], a table specifying
  the preset aliases.

src/cmd/ksh93/include/shtable.h,
src/cmd/ksh93/sh/init.c:
- Rename inittree() to sh_inittree() and make it extern, because we
  need to use it in main.c (sh_main()).

src/cmd/ksh93/sh/main.c: sh_main():
- Init preset aliases from shtab_aliases[] only if the shell is
  interactive and not in POSIX mode.

src/cmd/ksh93/bltins/typeset.c,
src/cmd/ksh93/tests/alias.sh:
- unall(): When unsetting an alias, pass on the NV_NOFREE attribute
  to nv_delete() to avoid an erroneous attempt to free a preset
  alias from read-only memory. See: 5d50f825

src/cmd/ksh93/data/builtins.c:
- Remove "history" and "r" entries from shtab_builtins[].
- Revert changes to inline fc/hist docs in sh_opthist[].

src/cmd/ksh93/bltins/hist.c: b_hist():
- Remove handling for 'history' and 'r' as builtins.

src/cmd/ksh93/sh.1:
- Update accordingly.

Resolves: https://github.com/ksh93/ksh/issues/125

NEWS

@@ -17,6 +17,11 @@ Any uppercase BUG_* names are modernish shell bug IDs.
      b. 'command' can now stop the shell from exiting if a command that it
 	invokes tries to modify a readonly variable (fixing BUG_CMDSPEXIT).
 
+- The 'history' (== 'hist -l') and 'r' (== 'hist -s') interactive shell
+  history commands have reverted to preset aliases and are now only loaded if
+  the shell is interactive and not initialised in POSIX mode. This avoids
+  unneeded conflicts with external commands by these names, particularly 'r'.
+
 2020-09-09:
 
 - Fixed BUG_LOOPRET2 and related bugs. The 'exit' and 'return' commands without