Handle Meta modifier in Kitty and XTMODKEYS. Add Meta
indicator to notcurses-input. Update man page. Add
NCKEY_META_{SHIFT, CTRL, ALT, META} constants. Add
"modifiers" field to ncinput struct. Add inline
functions for testing modifiers. Remove special-casing
in Kitty protocol that capitalized all lowercase ASCII
when ctrl was pressed; we don't do this for XTMODKEYS.