mnemonic 'ENTER' with zcurses

[Ray Andrews <rayandrews@xxxxxxxxxxx>]

This is a pretty trivial matter but 'zcurses input' returns all sorts of 
helpful mnemonics like 'NPAGE', 'DOWN', 'PRESSED1' and so on.  Where are 
these defined?  No likely headers here than I can see.  Closest I can 
find is:

 https://www.man7.org/linux/man-pages/man3/getch.3x.html

... which seems a very similar list.  However that page includes 'ENTER' 
for the predictable thing, but zcurses seems to return no mnemonic for 
that, it's ugly old: " $'\n' ".   Just as a matter of curiosity I'd like 
to know what's going on there.  I'd prefer the mnemonic if it could be had.