avoid calling exit with a constant; use EXIT_* instead

avoid calling exit with a constant; use EXIT_* instead

This appeases a new gnulib-provided "syntax-check".
* daemon/libvirtd.c (main): Use EXIT_FAILURE, not 1.
* proxy/libvirt_proxy.c (main): Likewise, and EXIT_SUCCESS, not 0.
* tests/conftest.c (main): Likewise.
* tests/reconnect.c (main): Likewise.
* tests/testutils.h (EXIT_AM_SKIP): Define.
* tests/nodeinfotest.c (mymain): Use EXIT_AM_SKIP, not 77.
* tests/qemuargv2xmltest.c: Likewise.
* tests/qemuxml2xmltest.c: Likewise.
* tests/virshtest.c (mymain): Likewise.