Back port ksh93v- float, int, and exp10 changes from math.tab (#299)

src/cmd/ksh93/data/math.tab:
- Added exp10().
- Remove int() as being an alias to floor().
- Created entries for local float() and local int() which are
  defined in features/math.sh.

src/cmd/ksh93/features/math.sh:
- Backport floor() and int() related code from ksh93v-.

src/cmd/ksh93/sh.1:
- Sync man page to math.tab's potential functions.

NEWS

@@ -3,6 +3,13 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2021-05-07:
+
+- Backported three ksh 93v- math.tab changes, allowing for an exp10()
+  arithmetic function if one exists in the C library, a new float()
+  function, and lastly an updated int() function that rounds to zero
+  instead of being an alias to floor().
+
 2021-05-05:
 
 - Fixed: a preceding variable assignment like foo=bar in 'foo=bar command'