vtls: Use bool for Curl_ssl_getsessionid() return type

The return type of this function is a boolean value, and even uses a
bool internally, so use bool in the function declaration as well as
the variables that store the return value, to avoid any confusion.
tool_help: Use camel case for UNIX sockets feature output

In line with the other features listed in the --version output,
capitalise the UNIX socket feature.
sockfilt.c: Replace 100ms sleep with thread throttle

Improves performance of test cases 574 and 575 by 50%.

A value of zero causes the thread to relinquish the remainder
of its time slice to any other thread of equal priority that is
ready to run. If there are no other threads of equal priority
ready to run, the function returns immediately, and the thread
continues execution.

http://msdn.microsoft.com/library/windows/desktop/ms686307.aspx
sockfilt.c: Reduce the number of individual memory allocations

Merge multiple internal arrays into one, even if some variables
will not not be used. They are all created with the number of
file descriptors as their size.

Also fix possible thread handle leak in CloseHandle-loop.