Allow switching between different log methods at runtime.

Allow switching between different log methods at runtime.

Currently, para_client defines its para_log function via the
INIT_STDERR_LOGGING() macro which generates a log function that
writes to stderr. However, we will need to switch to a different,
curses-aware logging function when operating in interactive mode.

To support more than one log method the type of para_log is changed
from a function to a (public) pointer variable. This variable is
supposed to point to the log function currently in use so that the
application can simply set para_log differently in order to switch
between log functions.

The patch also changes the INIT_STDERR_LOGGING() macro to receive the
name of the log function to be defined and to let para_log point to
the newly defined function.