Fix default dump file permission : 0666 => 0600

[Etsukata]

Currently, the default dump file permission is 0666. But there will be
no reason to let other users to read or modify dump files.

An attacker could inject some malicious escape sequences into a file,
which may be executed on a victim’s terminal emulator (Not Critical).

[Etsukata]

• Ref
  • SEI CERT : FIO06-C. Create files with appropriate access permissions
    • https://wiki.sei.cmu.edu/confluence/display/c/FIO06-C.+Create+files+with+appropriate+access+permissions

[Etsukata]

This PR does not mean the current implementation is vulnerable to symlink attacks. Because the sticky bit is usually set on /tmp and sigdump uses just /tmp (not sub dirs of '/tmp').