lib/salt.c: gensalt(): Default to SHA512 instead of DES

[alejandro-colomar]

DES is insecure; use it only if explicitly requested.

• Closes: Fallback to DES when ENCRYPT_METHOD is unset is insecure: propose SHA512 or safe default #1278
• Reported-by:@andreboscatto
• Cc: @ikerexxe

---

Do not merge before the release of 4.20.

Revisions:

v1b

• Rebase

$ git rd 
1:  f4dd13ccb108 < -:  ------------ */: Remove support for MD5_CRYPT_ENAB
2:  85771ad90a6c < -:  ------------ lib/salt.c: Compact conditionals
3:  fdc32380c44a < -:  ------------ */: chpasswd(8): -m,--md5: Remove option
4:  43b5bef9f464 < -:  ------------ */: chgpasswd(8): -m,--md5: Remove option
5:  5bb41add8c8a = 1:  0c12025560a2 lib/salt.c: gensalt(): Default to SHA512 instead of DES