LinuxKPI: Improve sysfs support.

• Add functions for creating and merging sysfs groups.
• Add sysfs_streq function to compare strings ignoring newline from the sysctl userland call.
• Add a call to sysfs_create_groups in device_add.
• Remove duplicate header include.
• Bump __FreeBSD_version.

Reviewed by: hselasky
Approved by: imp (mentor), hselasky
MFC after: 4 days
Differential Revision: D21542