mg.c: White-space only

Properly indent code that is interior to a block
perl.h: Move a string constant to a #define

This is so it can be used in another file
Fix HP-UX $! failure

HP-UX strerror() returns an empty string for an unknown error code.
This caused an assertion to fail under DEBUGGING builds.  This patch
removes the assertion and changes the return into  a non-empty string
indicating the errno is for an unknown error.
mg.c: White-space only

Indent code that's in a block added in the previous commit.