Import unmodified expat 2.0.0.
Initial version of available features support.
Add README.AVAIL.
Mention docs.
Build SHA-1 support for checksumming.
Merge gdb_byte fix from HEAD.
Add RAM caching, checksum, and non-sequential register support.
Cache items read from the target this session, even without checksums.
Compile in the XML DTD.
Handle XInclude processing as a separate pass.
Doc update from last commit.
	* README.gdb: New file.
Require targets to provide all their registers if an XML description
is used; fix various bugs this uncovered.  Allow ARM descriptions without
FPA.
Update g/G packet support.
Basic register group support.
Documentation update.
Add user documentation (almost entirely written by Jim Blandy).
Add menus for the new text.
Update based on Eli Zaretskii's suggestions:
- Fix reference to qPart packet description.
- Fix spacing in itemized lists and around examples in info.
- Rephrase explanation of how multi-annex descriptions are retrieved.
- Provide more detail on "SHA-1".
Remove readonly; implement save-restore.
Make the last few reggroup changes compile.