truncating string copy

The majority of our calls to strncpy are in the form
  (void) strncpy(dst, src, n);
  dst[n] = '\0';
so add a new routine, copynchars, which does that.  A few calls care
about strncpy's return value and at least one relies on it only copying a
substring without also terminating the output, but most don't care about
either and none seem to care that `n' ought to have type size_t instead of
int.  The new routine matches our usage better, but I haven't gone through
to change the existing strncpy calls.

include/extern.h

@@ -807,6 +807,7 @@ E char *FDECL(upstart, (char *));
 E char *FDECL(mungspaces, (char *));
 E char *FDECL(eos, (char *));
 E char *FDECL(strkitten, (char *,CHAR_P));
+E void FDECL(copynchars, (char *,const char *,int));
 E char *FDECL(s_suffix, (const char *));
 E char *FDECL(xcrypt, (const char *,char *));
 E boolean FDECL(onlyspace, (const char *));