bug#50403: 27.2; calc-grab-region should check rectangle-mark-mode

[Howard Melman]

calc-grab-region should check if rectangle-mark-mode is
non-nil and if so it should behave as calc-grab-rectangle
otherwise it should work as it does.

This would allow a single keybinding of calc-grab-region to
serve both purposes for those users that use
rectangle-mark-mode. 

In GNU Emacs 27.2 (build 1, x86_64-apple-darwin18.7.0, Carbon Version 158 
AppKit 1671.6)
of 2021-03-27 built on Traviss-Mac.local

-- 

Howard