(paths): Redefine as type pathlike.
(display_error): Declare a few different ways to allow more flexible usage
throughout.
(display_error_fmt): New function for those cases where C++ just isn't enough.
(add_path): Rewrite to allocate pathlike paths. Always tack on a trailing
slash since that's what everyone who uses it wants. NULL terminate the path
list. Register "it's a system path" field.
(init_path): Call add_path with info regarding whether path is a system path or
not.
(pathlike::check_existence): Move into pathlike class. Accept file and
extension arguments to build up path on the fly. Modify other arguments.
(find_on_path): Constify return value and appropriate arguments. Eliminate
short-circuit for fully-qualified paths. Simplify loop which iterates over
path.
(already_did): Constify argument.
(track_down): Ditto. Regularize some error messages.
(find_app_on_path): Ditto.
(cygcheck): Constify argument. Use 20th century string handling functions.
(dump_sysinfo): Remove odd inclusion of '\\bin' "Just in case". Accommodate
change of paths to pathlike.
* path.cc (isslash): Rename from SLASH_P and use throughout.
(rel_vconcat): Front-end to vconcat which prepends cwd to path before passing
along for conversion to native windows.
(cygpath): Remove "./" test. Call rel_vconcat if filename appears to be
relative.
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