Remove one level of quoting when spawning commands

Change spawn_command_line_sync to spawn_sync to reduce the level of
quoting required when passing commands to the function.  Since
spawn_command_line_sync uses the parent's environment, the replacement
call also uses the parent's environment.

This change permits unmounting of file systems mounted with an
apostrophe in the mount point name.

Ubuntu launchpad bug 151796 - Can't unmount partition with apostrophe
                              in label
https://bugs.launchpad.net/ubuntu/+source/gparted/+bug/151796