Reset SIGPIPE handler to default in preexec

Python sets SIGPIPE to ignored and doesn't set it back when creating a new
process, resulting in things like 'cat /dev/zero|head -c5' failing in mock
shell, even though they should succeed.  This sets the signal back to default in
util.do's preexec_fn.
Drop redundant requires lvm2-python-libs
Revert specfile changes (pyliblzma, symlinks)
Remove bundled C nofsync plugin
Remove both DNF plugins

The nofsync DNF plugin is superseded by the nosync library. The noverify plugin
seems to give only marginally small speedup (~1s) when nosync is used. Therefore
removing it because it's not worth the complications arising from modifying DNF
internals.
Implement support for using Mikolaj's nosync library

It is disabled by default, to enable it use config_opts['nosync'] = True