lib/util.cpp had become a landfill of unrelated stuff
    - move the process-related code to lib/proc_control.cpp
    - move file-related code to lib/filesys.cpp
    - move client mutex code to client/main.cpp

The client has 2 unrelated types of mutual exclusion:
thread level (between the main and throttle threads)
and process level (prevent 2 clients from running on a host).
    - Rename "client_mutex" to "client_thread_mutex".

- parse.h: don't declare boinc_is_finite(); include the .h file

The client runs two "detection processes":
- GPU detection
- (Mac) check if in CPU emulation mode
The client was waiting indefinitely for these to exit.
If for some reason they don't exit, the client hangs.

Fix: wait no more than 10 sec for them to exit.
If still running, kill them and move on.

I took the opportunity to clean up the process-related code:

- add a time arg to get_exit_status().
    If zero, wait indefinitely for the child to exit.
    Else wait no more than that amount.
- get_exit_status() return an error code;
    the exit status is returned via a parameter.
- run_program() no longer takes a time argument.
    If you want to check for early exit, use get_exit_status()
- define PROCESS_ID as HANDLE (Win) or int (other)
    so we can unify the process interface
- remove process_exists().  It wasn't used anywhere,
    and get_exit_status(... 0) does the same thing.
- rename kill_program() to kill_process()
- don't use "prog" when you mean "pid"

get_exit_status(): dt<= means wait indefinitely; dt=0 means non-blocking

Note: the unit tests don't really test anything

this PR is getting to big.

Getting boinccas to build required some stuff I don't understand

This required changing a bunch of project files,
and was turning into a rabbit hole.

The VS header files have a conflict:
various structures (such as STRING) are defined differently
in ntsecapi.h than in winternl.h

This error appears off and on, for reasons I don't understand.
Changes in lib/str_util.h seem to trigger it.
Earlier in this PR is was happening.
I found a workaround on stackoverflow.com and added it.

But now the error is not happening, even without the workaround.
So, for now, I'm commenting out the workaround.
It's there if we need it in the future.