When executed from cron, commandtest would fail to correctly
identify daemon processes. Set session ID and process group
IDs at startup to ensure we have a consistent environment to
run in.

* tests/commandtest.c: Call setsid() and setpgid()

* src/fdstream.c (virFDStreamOpenFile, virFDStreamCreateFile):
Use VIR_FORCE_CLOSE instead of close.
* tests/commandtest.c (mymain): Likewise.
* tools/virsh.c (editFile): Use virCommand instead of system.
* src/util/util.c (__virExec): Special case preservation of std
file descriptors to child.

* src/util/command.c (virCommandRun): Don't capture output on
daemons.
* tests/commandtest.c (test18): Expose the bug.
Reported by Laine Stump.

* tests/commandtest.c (mymain): Kill off any leaked-in fds.
* autobuild.sh: Don't leak fds.
Signed-off-by: NEric Blake <eblake@redhat.com>

* tests/Makefile.am (SUBDIRS): Add commanddata.
* tests/commandtest.c (checkoutput): Delete correct file.
(test4): Delete pid file.
(mymain): Delete unused variable.
* tests/commanddata/Makefile.am: New file.
* configure.ac (AC_OUTPUT): Build new makefile.
Reported by Dominik Klein.

Guarantee that outbuf/errbuf are allocated on success, even if to the
empty string.  Caller always has to free the result, and empty output
check requires checking if *outbuf=='\0'.  Makes the API easier to use
safely.  Failure is best effort allocation (some paths, like
out-of-memory, cannot allocate a buffer, but most do), so caller must
free buffer on failure.

* docs/internals/command.html.in: Update documentation.
* src/util/command.c (virCommandSetOutputBuffer)
(virCommandSetErrorBuffer, virCommandProcessIO) Guarantee empty
string on no output.
* tests/commandtest.c (test17): New test.

Most leaks could only occur on error cleanup paths.

This introduces a new set of APIs in src/util/command.h
to use for invoking commands. This is intended to replace
all current usage of virRun and virExec variants, with a
more flexible and less error prone API.

* src/util/command.c: New file.
* src/util/command.h: New header.
* src/Makefile.am (UTIL_SOURCES): Build it.
* src/libvirt_private.syms: Export symbols internally.
* tests/commandtest.c: New test.
* tests/Makefile.am (check_PROGRAMS): Run it.
* tests/commandhelper.c: Auxiliary program.
* tests/commanddata/test2.log - test15.log: New expected outputs.
* cfg.mk (useless_free_options): Add virCommandFree.
(msg_gen_function): Add virCommandError.
* po/POTFILES.in: New translation.
* .x-sc_avoid_write: Add exemption.
* tests/.gitignore: Ignore new built file.