Make GParted recognise reading blank file system labels (#685656)

GParted doesn't notice when a file system label is changed to blank.
GParted first calls the file system specific read_label() method.  When
the label is blank read_label() correctly sets partition.label to the
zero length string.  Second GParted_Core::set_device_partitions() treats
the zero length string to mean that the label is unset and calls
FS_Info::get_label() to retrieve it from the cache of blkid output.
Blkid also doesn't notice when the file system label has been changed to
blank so reports the previous label.  Hence GParted displays the
previous file system label.

Fix by making label a private member variable of the class Partition and
providing access methods set_label(), get_label() and label_known()
which track whether the label has been set or not.  This only fixes the
fault for file systems which use file system specific commands to read
the label and when these tools are installed.  Otherwise GParted uses,
or has to fall back on using, the buggy blkid command to read the file
system label.

NOTE:
Many of the file system specific read_label() methods use a tool which
outputs more than just the label and use Utils::regexp_label() to match
leading text and the label itself.  If the surrounding text changes or
disappears altogether to indicated a blank label, regexp_label() doesn't
match anything and returns the zero length string.  This  is exactly
what is required and is passed to set_label() to set the label to blank.

Bug 685656 - GParted doesn't notice when file system label is changed to
             blank