Look for a PT_GNU_RELRO section and, if present, mprotect that range

Look for a PT_GNU_RELRO section and, if present, mprotect that range
instead of the [__got_start, __got_end) range.

Also, instead of mprotecting the [__plt_start, __plt_end) range,
just scan for sections which are both writable and executable and
mprotect them to read-only.  (This part was stolen from kettenis@)

ok kettenis@