Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

- Signal that data should not be modified.

- Move csup away from contrib/ and into usr.bin/. Software is no longer
contributed, and main development is happening in the FreeBSD repo.

Suggested by: joel

- Add proper error checking and printing to the CVSMode code when reading and
writing from/to streams, as leaving them out stops csup from cleaning up on
SIGINT and friends properly.

MFC after: 1 week

Merge support for CVSMode (aka. mirror mode) into csup. This means csup can now
fetch a complete CVS repository. Support for rsync update of regular files are
also included, but are not yet enabled. The change should not have an impact on
existing csup usage, as little of the existing code has changed.

- Update copyright.

- Add an optimization when parsing rcsfiles when the intention is to only send
details to the cvsup server. The deltatext does not need parsing, and some
parts of the rcsfile data structure doesn't need to be set up.
- Fix a bug where the RCS expansion mode is not written out.

A few bugfixes:
- Instead of including protocol info in diffs, strip them before adding
deltatext and take this into account when applying the diff later.
- Don't use strlen when the string in the RCS file may contain garbage. This got
caught in the checksumming before, but was not fixed until now. Instead of
using strlen, pass the token length when adding log and text entries to a
delta. Add an extra length parameter to duptext() to record the token length.
- When adding new branches to a file, add them in at the tail instead of the
head of the list to get correct ordering when writing out.
- Input stream when diffing was opened twice.
- Don't expand keywords in diffs between deltas.

- Import csup work from p4.