keys: graphical representation of linux control keys

Represent some control combinations by recognisable symbols for function
keys equivalent in most terminal environments as they are commonly used.
May help people discover easier access.

Enter, tab, and escape should match for all current applications (except
vimperator for an insignificant alias).  Delete and backspace represent
VT100/xterm and ASCII characters, but are different in VT220-based emulators
<http://www.macfreek.nl/memory/Backspace_and_Delete_key_reversed>.
In such cases ^h is distinct from backspace, but likely still works the same
for current occurrences where this is the intended behaviour.

In cases this does not work, the original bindings can still be easily
ascertained from surrounding keys, so should not pose much of a problem.