moved most OS-dependent functions in OS.cpp

Cygwin is GONE!

- usleep takes microseconds
- work on c solver interf

"pure windows" version (without cygwin) is almost there: the solver
interface now works quite nicely using native Windodiows sockets and
the timers work too. There are still some bugs with binary files and
user-created functions in the parser.

replaced poll() with direct call to select(). This should help in getting
the solver interface eventually work on Windows without cygwin

add button in GUI to choose if we should always listen to incomming
connections requests

added example of interactive command line driver for Gmsh

- rename CLIENT_VIEW into CLIENT_MERGE_FILE
- new CLIENT_PARSE_STRING

characteristic lenghts, that would lead to HUGE meshes (this is one
of the most common causes for erroneous bug reports on the mailing list)