tests: Run IPC with use-filesystem-sockets active (#455)

• tests: Run IPC with use-filesystem-sockets active

Provide an LD_PRELOAD library that simulates the presence
of /etc/libqb/use-filesystem-sockets so that we can test
that functionality without actually having the file on
the system and affecting everything else running on the
box.

• use $() rather than ``

• Cope with spaces in directory names

• Docs: quote DOXYGEN2MAN in Makefile.am rather than configure.ac