fix updating of line-offsets output info in plain output mode reviewed by tom jones

add support for classic diff(1) output; patch by Tom Jones

add a missing include for uint8_t and switch from <inttypes.h> to <stdint.h> ok millert stsp

move headers from include/diff/ to include/; makes things easier for consumers

check return value of diff_output_lines

optionally provide information about the generated diff to the API user

move some definitions from the public diff_main.h to an internal header file

avoid include statements in header files; make each .c pull in what's needed

replace enum diff_rc errors with plain errno values

KNF: use max 80 columns

KNF: function declaration return types on separate line

remove code dup of diff_main() invocation for plain vs unidiff

Only print (unified) header in unified format.

initial commit