Option to turn down verbosity about b2

[Kojoley]

I would also considered this a performance issue because currently virtual-targets.register is a huge bottle-neck, 3568793 out of 3687209 (96.8%) calls are to register these headers.

[grafikrobot]

There are two issues in this:

• Silencing the commands (and their output) for which we have -d0 .
• Having an action per file copy to do things like header file installs. For that there's this boostorg/build#454 old issue.

[pdimov]

I think that this is why I made --with-headers work, so that one can two-phase install as in

b2 -d0 --with-headers install
b2 install

[Kojoley]

I think 1 file(s) copied. output should be always suppressed (I have a PR for the same thing in tests #390).

copy action could be defined with quietly modifier like mkdir already does so common.copy ... lines will appear only with -d+2.