termkey_keyname2sym() looks up the symbolic key value represented by the given string name. This is a
case-sensitive comparison. If the given name is not found, TERMKEY_SYM_UNKNOWN is returned instead. This
function is the inverse of termkey_get_keyname(3), and is a more specific form of
termkey_lookup_keyname(3) which only recognises names as complete strings.
Because the key names are stored in an array indexed by the symbol number, this function has to perform a
linear search of the names. Use of this function should be restricted to converting key names into
symbolic values during a program's initialisation, so that efficient comparisons can be done while it is
running.