maintain and is only going to get worse as more are used. So instead, add a new
uint64_t member to cmd_entry which is a bitmask of upper and lowercase options
accepted by the command.
This means new single character options can be used without the need to add it
explicitly to the list.
crashes when trying to find the new active pane.
While here, nuke an unused pane flag.
Fixes PR 6160, reported by and a slightly different version of diff tested by
ralf.horstmann at gmx.de.
