mount: fix parsing offset= followed by more options

mount: fix parsing offset= followed by more options

mount does not parse parse offset= option if it is followed by other
options.  In umount, the parsing is done with the get_value() function.
This patch moves get_value to fstab.c (with new name get_option_value())
and fixes mount to use the function.

[kzak@redhat.com: - rename to get_option_value()
		  - use fstab.c rather than sundries.c]

Novell bugzilla: #666150

Reported-by: Ludwig Nussel <ludwig.nussel@suse.de>
Signed-off-by: Petr Uzel <petr.uzel@suse.cz>
Signed-off-by: Karel Zak <kzak@redhat.com>