No need to repeat common code.

* bootstrap.conf (gnulib_modules): Import calloc-posix.
* src/util/bridge.c (brInit): Use virSetCloseExec.
(brSetInterfaceUp): Adjust flags name.
* src/uml/uml_driver.c (umlSetCloseExec): Delete.
(umlStartVMDaemon): Use util version instead.