…block (#840)

GuidelineWriter::write() used preg_replace() with user-provided guidelines
as the replacement string. PHP's preg_replace() interprets $N and \N in
replacement strings as backreferences — since the pattern has no capture
groups, any $99, $1, \1, etc. in user guidelines silently evaluated to
empty string on every boost:update run.

Fix: use preg_replace_callback() so the callback return value is treated as
a literal string with no backreference interpretation.

Adds two regression tests covering $N dollar amounts and backslash patterns
in guidelines content during the in-place replace path.

Closes #800 (Bug 2)