Allow configuration of the signals used for GC

On Posix systems druntime's GC uses SIGUSR1 and SIGUSR2 to implement
suspension of threads for its garbage collector. When integrating D into
existing programs written in other languages such as C or C++,
SIGUSR1/SIGUSR2 may already have an existing meaning. This allows those
programs to use currently unused signals for D's GC, most likely some of
the POSIX real time signals where availble.

This is acheived by calling thread_setGCSignals prior to thread_init, at
most once. If it is not called the runtime will continue to use SIGUSR1
and SIGUSR2. The functionality leaves the choice of signals up to the
caller, it does not assume any particular real time signal or that real
time signals are available.
Fix identation
localize some imports
Revert "Issue 7942 - Appending different string types corrupts memory"

This reverts commit d0f8bca7ad1eea1b8e9e2774fc84a1af7ce73815.
Merge pull request #953 from yebblies/revert7942

Revert "Issue 7942 - Appending different string types corrupts memory"
Add documentation comment and missing import for Android
boilerplate mixin
Merge pull request #706 from adamdruppe/windows_dll

boilerplate mixin
Merge pull request #954 from joakim-noah/tls_docs

Add documentation comment and missing import for Android
Add INVALID_FILE_ATTRIBUTES to core.sys.windows.windows
Merge pull request #955 from tom-tan/add-windows-INVALID_FILE_ATTRIBUTES

Add INVALID_FILE_ATTRIBUTES to core.sys.windows.windows
add special c_long_double type for MSVC++
Merge pull request #958 from WalterBright/c_long_double

add special c_long_double type for MSVC++
Merge pull request #919 from MartinNowak/localizeImports

localize some imports
fix property enforcement
Improve documentation of thread_setGCSignals.

Fix some typos, and ensure that the documentation is available when
generating docs on any system, while also clarifying it is Posix-only.
Merge pull request #888 from lcapaldo/configurable_signals

Allow configuration of the signals used for GC
Merge pull request #959 from 9rnsr/enforceProp

fix property enforcement