doc: Remove sed-based renaming, just use doxygen's built-in preprocessing

Doxygen's latex escaping vary a lot with the context and the doxygen version
(one or two \- around each \_ and sometimes between capitals too).
Don't bother applying many complex regexps with wildcards, it doesn't scale.
doxygen can do it early for us.

(cherry picked from commit 1f46384068f36d507b75ae8031a6332140d91c90)
dist: fix support for doc/doxygen-doc being a symlink

make dist works when building out-of-source if $srcdir/doc/doxygen-doc
is a symlink to builddir/doc/doxygen-doc.
But it actually fails to copy the doxygen-doc tree because it
doesn't dereference the symlink. Fix that.

(cherry picked from commit 0ebeff689e9414d5eedbf53e7c8697a3af5e4b72)