maint: avoid -Wstringop-truncation warnings from upcoming GCC8

maint: avoid -Wstringop-truncation warnings from upcoming GCC8

* src/create.c (start_private_header, start_header): Convert
trivial uses of strncpy to memcpy, to avoid warnings like this:
In function 'strncpy',
    inlined from 'start_private_header' at create.c:522:3:
/usr/include/bits/string_fortified.h:106:10: warning: \
  '__builtin_strncpy' output truncated before terminating nul \
  copying 2 bytes from a string of the same length \
  [-Wstringop-truncation]