Create um_flags in the ufsmount structure to hold flags for a UFS filesystem.
Convert integer structure flags to use um_flags:

int um_candelete; /* devvp supports TRIM */
int um_writesuspended; /* suspension in progress */

become:

#define UM_CANDELETE 0x00000001 /* devvp supports TRIM */
#define UM_WRITESUSPENDED 0x00000002 /* suspension in progress */

This is in preparation for adding other flags to indicate forcible
unmount in progress after a disk failure and possibly forcible
downgrade to read-only.

No functional change intended.

Sponsored by: Netflix