Address a very theoretical fringe case.  We implicitly assume that chars
are of uniform size on all systems, specificly that they are 8 bits wide.
So, make that assumption explicit.

One problem is that our data files and network streams specify lenght
values in multiples of chars while actually transmitting a specific number
of bits.  On systems with e.g. 32 bit chars, we can not expect or rely on
OS / libs to convert between 8 and 32 bits.

* subversion/include/private/svn_dep_compat.h
  (): Error out if chars are defined as something other than 8 bits.
      Allow for skipping that check by defining SVN_ALLOW_NON_8_BIT_CHARS.

* notes/knobs
  (SVN_ALLOW_NON_8_BIT_CHARS): Document this new compile time option.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1627962 13f79535-47bb-0310-9956-ffa450edef68