Add System_Parse_CommandForUnix to the KWSys System interface as a
utility to parse a unix-style command line.  Move the existing
implementation out of ProcessUNIX.  Add a flags argument reserved for
future use in providing additional behavior.

This helps avoid fixing VMS-specific code for non-VMS compilers where it
isn't needed anyway.

COMP:Fixed warning with gcc 4.3.3: passing argument 1 of kwsysProcessSetVMSFeature discards qualifiers from pointer target type.

This achieves basic process execution on OpenVMS.  We use work-arounds
for different fork()/exec() behavior and a lack of select().

VMS emulates fork/exec using setjmp/longjmp to evaluate the child and
parent return cases from fork.  Therefore both must be invoked from the
same function.

Since select() works only for sockets we use the BeOS-style polling
implementation.  However, non-blocking reads on empty pipes cannot be
distinguished easily from the last read on a closed pipe.  Therefore we
identify end of data by an empty read after the child terminates.

This avoids use of an uninitialized value in the KWSys ProcessUNIX
polling implementation when no timeout is given.

ENH: check in almost building VMS stuff with VMSBuild directory since the bootstrap script will not work on VMS

COMP:Fixed gcc 4.3.2 warning with -O1 and above: ignoring return value of read'), declared with attribute warn_unused_result

According to "man select" on Linux it is possible that select() lies
about data being ready on a pipe in some subtle cases.  We deal with
this by switching to non-blocking i/o and checking for EAGAIN.  See
issue #7180.

ENH: all ctype function have the same issue: char can be signed or unsigned, since isspace only deal with >=0 value (except EOF) one has to first cast it to unsigned char

BUG: Fix hang in Process_Kill on OS X caused by an OS bug in which a pipe read end cannot be closed if the pipe write end is open, the pipe is full, and another process is blocking waiting to write.  Work around the problem by killing the children before closing the pipes.

BUG: Do not send both SIGSTOP and SIGKILL when killing a process.  The SIGSTOP seems to be able to block the SIGKILL occasionally.  Also the SIGKILL is sufficient since the process table entry will still exist until it is reaped with waitpid.

ENH: Changes based on patch from Ryan C. Gordon to enable process execution on BeOS.  There seems to be no way to implement it without polling (or threads).

ENH: Adding tests KWSYS_C_HAS_PTRDIFF_T and KWSYS_C_HAS_SSIZE_T to help ProcessUNIX.c build everywhere without warnings.

ENH: Added Process_SetPipeNative method to allow user code to override the pipes connected to the child pipeline.

COMP: Use SA_RESTART only if it is defined for the current platform.  This partially addresses bug#3556.

ENH: Implemented handling of SIGCHLD to detect the termination of immediate children.  This allows grandchildren to remain running after the children exit.

BUG: When more than one command is given and one of them fails to start and the rest are killed, do not forget to reap the killed children.