Fix configure / build system

Don't add X11 stuff when --without-x.
Enable compilation without FEATURE_PERSIST
Fix 'help :s'

In order to make :suspend work, 's' had been added to the internal
command list so that :s is still a valid abbreviation for :substitute.
It has been forgotten to add the necessary link in the documentation,
though.
Fix backslash handling

Even if not every \ at the beginning should result in a literal \
(but only in the case of \\, so that \( won't result in \(, but (,
and \$ won't result in \ and a variable substitution, but $), any
subsequent \\ must be treated as a quoted special character, namely \,
again.

(This condition has been changed from the previous version, where
subsequent \\ worked. As a side effect of the change, handling of
subsequent \\ has been broken.)
Moved my debian patch into the upstream code:
Elvis 2.2.0 has a bug causing it to exit 1 and not save any
files if started in a directory which contains a directory
"tags".
Complete the list of possible features

Add missing region and stdin.
Remove weird syntax assumption

Don't consider files in the temp directories to automatically be email.
Enhance anchoring by introducing an alternative way to specify the column

Anchoring two comments at the same column, when the comment keywords are
subsets or supersets of each other, will fail, because the shorter comment
keyword will always win. This is because the keywords are searched in
ascending order.

  Example:

  comment -
  comment --
  anchor 1 - --
  font a -
  font b --

  All lines starting with -- will be in font a, too!

To solve this problem, there is an alternative way to specify the column now.
A column number prefixed by a ~ character will be preferred to the same
column number without prefix.

  Example:

  comment -
  comment --
  anchor 1 -
  anchor ~1 --
  font a -
  font b --

  All lines starting with -- will be in font b now!

As a drawback, the maximum column number that can be used diminishes from
4094 (0xFFE) to 2046 (0x7FE).
Improve syntax highlighting for patches:

  * add comments ++, diff and @@ to color these lines
  * add special anchor ~1 for ++ and -- to differentiate from + and -
  * use normal font for file related information and keyword font for @@