View source for *Mview management of exec for virtual files

'''execvp''' needs a file in the file system. When the file is a virtual file there is no way to ''feed'' the file to '''execvp'''. The basic idea is to create a temporary copy of the executable file and then execvp it instead of the virtual file.

The support for exec is in '''um_exec.c'''.

There is also the binfmt support that needs to execute an interpreter for the executable instead of the executable, and the interpreter can be a virtual file, too.

Let us first consider the case of standard executables (without binfmt, see the '''wrap_in_execve''', at the end, the ''else'' branch of ''if (binfmtser != UM_NONE)'').

When the file is virtual (and the module does not need to give a specific implementation of execve) the executable is copied (function '''filecopy''') and the executed.
The virtual file temporary copy file will be deleted at the next system call executed by the same process (thus execve will be completed at that time).

When binfmt need to start an interpreter for the executable, *mview executes a wrapper named '''umbinwrap''' instead of the executable. '''umbinwrap''' encodes in argv[0] the path of the interpreter, the path of the executable and when binfmt needs it the former argv[0].

The wrapper decodes argv[0] and starts the interpreter, working as a standard process under the control of *mview.
'''umbinwrap''' uses '''execvp''' to start the interpreter thus it could be a virtual file or even a further interpreter, but um_exec can cope with both cases.