Implement option -v

The status report printed by default before is now only printed if -v is given.
Document -v
Refactor fdup.c

The following things were refactored:

 * renamed m to matcher
 * renamed print_walker to walker
 * renamed operation_mode to mode
 * made mode a local variable
 * added a comment to explain why there are global variables
 * corrected wrong formatting
Prototype functions in fdup.c
Do not include <sys/stat.h> in match.h
Set errno if file system does not support btrfs clone ioctl
Reword fdup.1
Put action code into action.c
Minor stylistic changes in match.c and match.h
Factor out -s parsing routine in fdup.c
Merge branch 'dev'
Update README