Partial forward compatibility fix for vibe-core.

Merge pull request #1084 from wilzbach/trailing-whitespace ...

Fix trailing whitespace within dub

Remove all trailing whitespace ...

> sed 's/[ \t]*$//' -i **/*.d

Add enforcement message.

Merge remote-tracking branch 'upstream/stable' into merge_stable ...

# Conflicts:
#	source/dub/compilers/compiler.d

Prevent hardLinkFile from unlinking a target just to then turn around and link the same source to the same target again (on Posix).

Fix vibe.d compatibility for vibe.d < 0.8.0.

Improve vibe.d compatibility. ...

If another package links against a vibe.d sub package instead of the base package, this now properly uses the vibe.d types instead of DUB's internal forks.

Fix compilation errors in the "library" configuration.

Add file/line to JSON parser error messages. Fixes #957.

Remove POSIX special-casing from copyFile ...

Since `std.file.copy` provides a means to preserve attributes of a file
as of DMD 2.067.0 or later, it is preferable to use this option rather
than having a custom-written solution.  `version (Posix)` special-casing
is therefore used only where the DMD version does not support it.

Note that where the `version (Posix)` code uses a `chown` call followed
by a `chmod` call, `std.file.copy` only uses a `chmod`.  This is not
expected to cause any issues but should be borne in mind as a possible
breaking change for some use-cases.

Fixes https://github.com/dlang/dub/issues/992.

trivial: Fix typos

Merge pull request #891 from MartinNowak/dmd_nightly ...

test with dmd-nightly and dmd-beta

Handle unbound stdout/stderr gracefully. See #892.

fix ambiguous isNumeric overload ...

- isNumeric!string matches std.string.isNumeric and std.traits.isNumeric

Eliminate use of Json.opDispatch. ...

(cherry picked from commit bf65c40)

Check return value of rawRead().

Use std.stdio instead of the deprecated std.stream. ...

Since opening non-ASCII filenames on Windows is fixed for the latest DMD version, it now only has to be made sure to use a recent compiler to build DUB.

Add assertion message.

Remove unused code that throws a warning on DMD 2.069.0-b2.

avoid early allocation and formatting of lazy message ...

- one of the biggest unneeded allocation sources in
  dub found while testing -profile=gc

Remove immutable casts where possible for Path. See #625.

Add some additional assertions to Path for #601.

Fix naming convention of FileMode enum. ...

Also some minor code style fixes.

Fix JSON unit test for 2.068.

Change Build(Options/Requirements) to BitFlags.

Enable file names/line numbers and field names for the updated JSON module.

Fix compilation of the updated vibe.d modules on all supported DMD versions.

Upgrade Json/serialization code to latest vibe.d version.

fix #511 - copyFiles should not overwrite identical files on windows