lib: Add thread-safety to global list of handles.

lib: Add thread-safety to global list of handles.

This commit uses the Gnulib 'lock' module to implement a mutex on
the global list of handles which is stored by the library.

Note that Gnulib nicely avoids explicitly linking with -lpthread
unless the application program itself links to -lpthread.  Locks
are only enabled in multithreaded applications.

$ ldd src/.libs/libguestfs.so.0.217.0
	linux-vdso.so.1 =>  (0x00007fffcb7ff000)
	libc.so.6 => /lib64/libc.so.6 (0x00007f96a4e6c000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f96a544d000)

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