add gc.config to configure profiling and pool limits through the command line
return Thread object in start method

- allows to write more concise code to start a Thread
Merge branch 'fixThreadDocs' into threadStartMethodChaining
Merge pull request #973 from MartinNowak/threadStartMethodChaining

return Thread object in start method
Merge pull request #817 from rainers/gc_config

make GC configurable through environment variable
Add CLOCK_REALTIME_FAST for FreeBSD.

Added the platform-specific CLOCK_ values for Linux and FreeBSD and put
them in their own files, deprecating the pre-existing ones in the posix
file.

This is required for Phobos pull# 2584.
Merge pull request #981 from jmdavis/CLOCK_REALTIME_FAST

Add CLOCK_REALTIME_FAST for FreeBSD.