Don't allow `got patch' to delete files that are not known, or add files that are already known and to edit files that are known, not obstructed and without conflicts.

add two helper functions (schedule_add, schedule_del) and move the guts of apply_patch into a new function `patch_file'. This simplifies apply_patch and makes easier to figure out what happens. Then, drop GOT_ERR_PATCH_PATHS_DIFFER since we allow the to rename files.

ok stsp

Allow the libexec helper, `got-read-patch` to be compiled, portably.