Implement the interceptors for popen(), pclose() and popenve()
functions. The first two are POSIX, the third one is specific
to NetBSD. popen() spawns a process and creates a FILE object piping
data from/to that process. pclose() closes the pipe and waits for
the process to terminate appropriately.
For the purpose of popen(), a new COMMON_INTERCEPTOR_PIPE_OPEN macro is
added that is like COMMON_INTERCEPTOR_FILE_OPEN, except it is passed
the new FILE* and no path.