* src/mkunitables.sh,src/unitable.*: Add.
* src/iso2022.c, src/iso2022.h: Use unitables.
* src/caps.c: Remove sequences for designating character sets.  Switch to using
	a second passed-in buffer for storing results in case the caller doesn't
	want substitutions done in-place.
* src/interpret.c: Perform substitution.
* src/table.c, src/table.h: Use gssize instead of ssize_t.
* src/termcap.c, src/termcap.h: Use gssize instead of ssize_t.
* src/utf8echo.c: Use strtol, allowing users to specify code points in hex
	format.