Add vasprintf() and asprintf()

The new utils.c functions vasprintf() and asprintf() are analogs to
sprintf() and vsprintf(). However, instead of requiring a buffer that is
large enough to hold the output data, the functions allocate a string
and return a pointer to it.

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>
Replace several uses of snprintf() by asprintf()

Use asprintf() in some cold code paths. While allocating memory on the
heap is a bit slower, using asprintf() is a bit more memory efficient
and less prone to buffer overflow errors.

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>
Refactor new_tempfile()

Avoid preallocating buffers on the stack, use dynamic memory allocation
instead. Also, change the semantics of new_tempfile() so that it returns
the full name of the temporary file and fix all call sites.

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>
config.c: Rework configuration serialization

Avoid preallocating buffers on the stack, use dynamic memory allocation
instead.

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>