Re-match '.' and '..' in tab completion (re: 5312a59d, aad74597)

Turns out there is a bona fide, honest-to-goodness use case for
matching '.' and '..' in globbing after all. It's when globbing is
used as the backend mechanism for file name completion in
interactive shell editors. A tab invisibly adds a * at the end of
the word to the left of your cursor and the resulting pattern is
expanded. In 5312a59d, this broke for '.' and '..'.

Typing '.' followed by two tabs should result in a menu that
includes './' and '../'. Typing '..' followed by a tab should
result in '../', (or a menu that includes it if there are files
with names starting with '..'). This is the behaviour in 93u+ and
we should maintain this.

To restore this functionality without reintroducing the harmful
behaviour fixed in the referenced commits, we should special-case
this, allowing '.' and '..' to match only for file name completion.

src/lib/libast/include/glob.h:
- Fix an inaccurate comment: the GLOB_COMPLETE flag is used for
  command completion, not file name completion. This is very clear
  from reading the path_expand() function in sh/expand.c.
- Add new GLOB_FCOMPLETE flag for file name completion.

src/lib/libast/misc/glob.c:
- Adapt flags mask to fit the new flag.
- glob_dir(): If GLOB_FCOMPLETE is passed, allow '.' and '..' to
  match even if expanded from a pattern.
- Clarify the fix from aad74597 with an extended comment based on
  <https://github.com/ksh93/ksh/issues/146#issuecomment-790991990>.

src/cmd/ksh93/sh/expand.c: path_expand():
- If we're in the SH_FCOMPLETE (file name completion) state, then
  pass the new GLOB_FCOMPLETE flag to AST glob(3).

Fixes: https://github.com/ksh93/ksh/issues/372
Thanks to @fbrau for the bug report.

NEWS

@@ -3,6 +3,11 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2021-12-13:
+
+- Fixed a bug introduced on 2020-08-09 that prevented '.' and '..' from
+  being completed when using file name tab completion.
+
 2021-12-11:
 
 - Fixed two more crashing bugs that occurred if ksh received a signal (such