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

replace enum diff_rc errors with plain errno values

fall back on file i/o in case an input file cannot be memory-mapped

KNF: use max 80 columns

diff cmdline tool: allow using Patience by -p arg

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

diff.c: move two #include to the top

myers: permit ~4Mb of state

switch default to unidiff

initial commit