fix part of realpath for drive-qualified DOS paths:

if a path or symlink target is drive-qualified, keep the drive letter
(this part from komh) and, if present, a leading (back)slash (from me)

missing: if a drive qualification is *not* followed by a (back)slash,
we must retrieve the per-drive cwd for the target drive and insert it
just like we insert the cwd for (normal/Unix) relative paths; maybe
consider redoing absolute/relative path logic, DOS paths are tristate

komh’s commit: fix realpath failure on OS/2

On OS/2, an absolute path is 'x:/path/to/file'. Because it has not
a leading slash, '/' is prepended, that is, '/x:/path/to/file'. As a
result, it fails to find a requested file.

misc.c

@@ -32,7 +32,7 @@
 #include <grp.h>
 #endif
 
-__RCSID("$MirOS: src/bin/mksh/misc.c,v 1.279 2017/08/07 21:39:25 tg Exp $");
+__RCSID("$MirOS: src/bin/mksh/misc.c,v 1.280 2017/10/10 21:30:43 tg Exp $");
 
 #define KSH_CHVT_FLAG
 #ifdef MKSH_SMALL
@@ -1783,6 +1783,24 @@ do_realpath(const char *upath)
 					/* keep them, e.g. for UNC pathnames */
 					Xput(xs, xp, '/');
 				}
+#ifdef MKSH_DOSPATH
+				/* drive letter? */
+				if (ctype(ip[0], C_ALPHA) && ip[1] == ':') {
+					/* keep it */
+					Xput(xs, xp, *ip++);
+					Xput(xs, xp, *ip++);
+					/* as well as a leading (back)slash */
+					if (mksh_cdirsep(*ip))
+						Xput(xs, xp, *ip++);
+					/*
+					 * XXX else: get the cwd on that drive
+					 * XXX and prepend it here as this is
+					 * XXX a drive-qualified relative path
+					 * XXX which we are supposed to convert
+					 * XXX to an absolute (with drive) one
+					 */
+				}
+#endif
 			}
 		}
 		/* otherwise (no symlink) merely go on */