Save XML files into a memory buffer rather than directly into a file

This introduces a "struct membuffer" abstraction that you can write
things into, and makes the XML saving code write to the memory buffer
rather than a file.

The UDDF export already really wanted this: it used to write to a file,
only to then read that file back into memory, delete the file, and then
*rewrite* the file after doing the magic xslt transform.

But the longer-term reason for this is that I want to try to write other
formats, and I want to try to share most helpers.  And those other
formats will need this memory buffer model.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

subsurface.pro

@@ -83,6 +83,7 @@ SOURCES =  \
 	gettextfromc.cpp \
 	libdivecomputer.c \
 	main.cpp \
+	membuffer.c \
 	parse-xml.c \
 	planner.c \
 	profile.c \