support gcc 5 builtins for multiplication overflow in safemult.h

fix copypasted comment in fmt_asn1dertag
add man pages for add_of, sub_of and assign

fmt/fmt_asn1dertag.c

@@ -3,7 +3,7 @@
 /* write int in least amount of bytes, return number of bytes */
 /* as used in ASN.1 DER tag */
 size_t fmt_asn1dertag(char* dest,unsigned long long l) {
-  /* encoding is either l%128 or (0x80+number of bytes,bytes) */
+  /* high bit says if more bytes are coming, lower 7 bits are payload; big endian */
   size_t n=0,i;
   unsigned long long t;
   for (t=l, n=1; t>0x7f; t>>=7) ++n;