Make ~ expand to home directory after unsetting HOME

There was an issue with tilde expansion if the HOME var is unset.

	$ unset HOME
	$ echo ~
	martijn

Only the username is returned. Users are more likely to expect the
current user's home directory as configured in the OS.

POSIXly, the expansion of ~ is based on the value of HOME. If HOME
is unset, the results are unspecified. After unsetting HOME, in
bash, ~ returns the user's home directory as specified by the OS,
whereas in all other shells, ~ expands to the empty string. Only
ksh93 returns the username. The behaviour of bash is more useful.

Discussion:
https://github.com/ksh93/ksh/pull/225#issuecomment-799074107

src/cmd/ksh93/sh/macro.c,
src/cmd/ksh93/tests/tilde.sh:
- sh_tilde(): Backport fix by Mike Gilbert from ksh2020.
  See:	https://github.com/att/ast/issues/1391
	https://github.com/att/ast/pull/1396
	070d365d
- Add test.

src/cmd/ksh93/COMPATIBILITY:
- Note this change.

src/cmd/ksh93/tests/tilde.sh

@@ -87,4 +87,12 @@ chmod +x $tmp/tilde
 nl=$'\n'
 [[ $($tmp/tilde foo) == "$PWD$nl$PWD" ]] 2> /dev/null  || err_exit 'tilde fails inside a script run by name'
 
+# ======
+# After unsetting HOME, ~ should expand to the current user's OS-configured home directory.
+unset HOME
+got=~
+[[ $got == /* && -d $got ]] || err_exit "expansion of bare tilde breaks after unsetting HOME (value: $(printf %q "$got"))"
+HOME=$tmp
+
+# ======
 exit $((Errors<125?Errors:125))