new function fdclose() in FreeBSD 11

[markvanatten]

As of FreeBSD 11.0, FreeBSD has its own function
fdclose(), which is declared in stdio.h and makes build fail:

9vx/a/portfns.h:120:7: error: conflicting types for 'fdclose'
void fdclose(int, int);
^
/usr/include/stdio.h:409:6: note: previous declaration is here
int fdclose(FILE *, int *);