syscall/: The non-standard interface exec() is now enshrined as a official...

syscall/:  The non-standard interface exec() is now enshrined as a official NuttX API.  I really dislike doing this but I think that this is probably the only want to load programs in the protected mode.  It is currently used by some example code under apps/ that generate their own symbol tables for linking.  Other file execution APIs relay on a symbol table provided by the OS.  In the protected mode, the OS cannot provide any meaning symbol table for execution of code in the user-space blob so that is they exec() is really needed in that build case.  And, finally, the interface is completely useless and will not be supported in the KERNEL build mode where the contrary is true:  An application process cannot provide any meaning symbolic information for use in linking a different process.