Calc; m d does not work

[Mustafa Kocaturk]

Please describe exactly what actions triggered the bug and the
precise symptoms of the bug.  If possible, include a backtrace by
doing `\[toggle-debug-on-error]', then reproducing the bug.

Even when in Deg mode, one has to use algebraic entry with unit of deg,
as follows:

m d
60            ==>      60
C             ==> cos -952.412980415e-3
alg' 60 * deg ==> cos 500.000000006e-3

Note the inexact result, which suggests there is internal conversion and
rounding error. The result should be exact, 0.5.
Moreover, the log should include which mode was in effect.

Emacs  : GNU Emacs 27.0.50 (build 1, x86_64-apple-darwin15.6.0, NS 
appkit-1404.47 Version 10.11.6 (Build 15G19009))
of 2018-04-05
Package: Calc