Fix handling of files without newlines in the head and tail builtins (#365)

The head and tail builtins don't correctly handle files that lack
newlines[*]:
    $ print -n foo > /tmp/bar
    $ /opt/ast/bin/head -1 /tmp/bar  # No output
    $ print -n 'foo\nbar' > /tmp/bar
    $ /opt/ast/bin/tail -1 /tmp/bar
    foo
    bar$
This commit backports the required changes from ksh93v- to handle files
without a newline in the head and tail builtins. (Also note that the
required fix to sfmove was already backported in commit 1bd06207.)

src/lib/libcmd/{head,tail}.c:
- Backport the relevant ksh93v- code for handling files
  without newlines.

src/cmd/ksh93/tests/builtins.sh:
- Add a few regression tests for using 'head -1' and 'tail -1' on a file
  missing and ending newline.

[*]: https://www.illumos.org/issues/4149

NEWS

@@ -15,6 +15,9 @@ Any uppercase BUG_* names are modernish shell bug IDs.
   return value that fits in a signed integer, typically a 32-bit value.
   Note that $? is truncated to 8 bits when the current (sub)shell exits.
 
+- The head and tail builtins now correctly handle files that do not have an
+  ending newline. (Note that the tail builtin is not compiled in by default.)
+
 2021-12-05:
 
 - Fixed an issue on illumos that caused some parameters in the getconf