spacemouse_udev.c: rename internal function 'device_in_use' to 'devnode_used_in_list'
spacemouse_udev.c: group static function together, remove function prototypes off of top of file
spacemouse_udev.c: use strdup instead of manual malloc
Revert "spacemouse_udev.c: use strdup instead of manual malloc"

This reverts commit fd6bc02fa118c72d8761878cb45b9df2482a0897.
libspacemouse.h/spacemouse.*: trim button event structure from buffer of spacemouse structure
spacemouse_evdev.c: do not use a variable for period
spacemouse_udev.c: remove_device() more readable and efficient