Move DEFAULT_REG_NAME constant to registers.h

Used to be in fileops.h.
Add comment to DEFAULT_REG_NAME constant
Extract append_path_to_expanded() in macros.c
Add clear_registers() function to registers unit
Use defines for blackhole and unnamed registers
Extract prepare_register() function in fileops.c
Add support of the %r (registers) macros
Document %r macros

Expands to register's content.
Add vim documentation tag for each supported macro
Document behaviour for invalid register name
Merge branch 'add-reg-macros'