libnotmuch.so.* linking fail on some environments. According to
David Bremner on irc:
"We jump through hoops with the linker script (notmuch.sym) so
the pragmas are not needed. And they are a little bizarre in a
library anyway..."
This pushes the error handling up one step, but makes the function
more flexible. Running out of memory still triggers an internal error,
in the spirit of other xutils functions.
We keep the lib/xutil.c version. As a consequence, also factor out
_internal_error and associated macros. It might be overkill to make a
new file error_util.c for this, but _internal_error does not really
belong in database.cc.
