Clean up format specifiers in log_error calls

This pull request cleans up compile warnings like:

iouyap.c: In function ‘foreign_listener’:
iouyap.c:420:9: warning: format ‘%d’ expects argument of type ‘int’, but argument 3 has type ‘ssize_t’ [-Wformat=]
         debug_log_fmt ("received %d bytes (sfd=%d)\n",
         ^

With this changeset, compiling no longer generates warnings (tested on gcc 4.8.2)