fix missing strerror text for EMULTIHOP

this is an obsolete error code from RFS, an obsolete predecessor of
NFS. POSIX documents it only as "Reserved", but maintains the
requirement that it be defined. as long as it is defined, it needs a
string for strerror to produce; the one chosen matches glibc and
documentation from other language runtimes I could find.

src/errno/__strerror.h

@@ -100,5 +100,6 @@ E(EREMOTEIO,    "Remote I/O error")
 E(EDQUOT,       "Quota exceeded")
 E(ENOMEDIUM,    "No medium found")
 E(EMEDIUMTYPE,  "Wrong medium type")
+E(EMULTIHOP,    "Multihop attempted")
 
 E(0,            "No error information")