Accept \kern and \mkern for math kerning

Also output the right one depending on whether the unit is "mu" or
not. Indeed, the two macros are identical, except that \mkern only
accepts unit mu, and kern does not accept it.

With this patch, both macros accept all units, but on writing beck,
the right one is used.

src/mathed/InsetMathKern.cpp

@@ -61,6 +61,8 @@ void InsetMathKern::write(WriteStream & os) const
 {
 	if (wid_.empty())
 		os << "\\kern" << ' ';
+	else if (wid_.unit() == Length::MU)
+		os << "\\mkern" << from_utf8(wid_.asLatexString()) << ' ';
 	else
 		os << "\\kern" << from_utf8(wid_.asLatexString()) << ' ';
 }

src/mathed/MathFactory.cpp

@@ -495,7 +495,7 @@ MathAtom createInsetMath(docstring const & s, Buffer * buf)
 		return MathAtom(new InsetMathMakebox(buf, true));
 	if (s == "makebox")
 		return MathAtom(new InsetMathMakebox(buf, false));
-	if (s == "kern")
+	if (s == "kern" || s == "mkern")
 		return MathAtom(new InsetMathKern);
 	if (s.substr(0, 8) == "xymatrix") {
 		char spacing_code = '\0';

src/mathed/MathParser.cpp

@@ -1763,7 +1763,7 @@ bool Parser::parse1(InsetMathGrid & grid, unsigned flags,
 			}
 		}
 
-		else if (t.cs() == "kern") {
+		else if (t.cs() == "kern" || t.cs() == "mkern") {
 			// FIXME: A hack...
 			docstring s;
 			int num_tokens = 0;