Also adds --with-uniscribe by default.

Part of https://github.com/harfbuzz/harfbuzz/issues/1728

Usermanual: Add new chapters.

Part of https://github.com/harfbuzz/harfbuzz/issues/1728

Usermanual-buffers-chapter: trim out fallback-of-ufuncs talk and just mention that stuff exists if you care to go find it.

Add a check for stringOffSet(uint16) overflow,
return early if overflow happens

[subset] Use iterators in glyf/loca subsetting

UCDN was ~120kb of data.  New implementatoin is 69kb in default builds,
and 49kb if built with HB_OPTIMIZE_SIZE or __OPTIMIZE_SIZE__.  The
latter automatically enabled if built with -Os or -Oz.

There's room to shave off another 10kb or 20kb.  That will follow later.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1652

[subset] Add morx, mort, kern, and kernx to the default layout tables…

Ignore gvar/MVAR/HVAR table
add support for --nameIDs=* option

Code is ugly.  Ugh.