lib/libcrypt: use explicit_bzero() to clear sensitive buffers

Prevent a potentially sufficiently smart compiler from optimising
away our attempts to clear sensitive buffers.

A related change was di

lib/libcrypt: use explicit_bzero() to clear sensitive buffers

Prevent a potentially sufficiently smart compiler from optimising
away our attempts to clear sensitive buffers.

A related change was discussed and rejected in D16059, but I don't
believe the reasoning there applies: the code clearly documents its
intent that the `memset` calls clear sensitive buffers so they don't
hang around. `explicit_bzero` is the appropriate function for this
purpose. A potential performance disadvantage seems less important:
the functions in crypt are specifically designed to be slow, so a
few extra calls to guarantee that sensitive buffers are cleared does
not significantly affect runtime.

See also: D16059
Reviewed by: delphij, kevans
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D47037

(cherry picked from commit a2c0d2026fb422ade2171da4bc6d5d2773b268a6)

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lib/libcrypt: another trivial style change

Normalize on hard tabs.

I didn't catch this before pushing the previous commit.

No functional changes intended.

MFC after: 2 weeks
MFC with: 8ef8da882ff

lib/libcrypt: another trivial style change

Normalize on hard tabs.

I didn't catch this before pushing the previous commit.

No functional changes intended.

MFC after: 2 weeks
MFC with: 8ef8da882ff475e3da3bde57d97593a68f7d97b2

(cherry picked from commit 61b15e6dfc963a0c67dbaeae7f4590674976111f)

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lib/libcrypt: remove trailing whitespace

No functional change intended.

MFC after: 2 weeks

(cherry picked from commit 8ef8da882ff475e3da3bde57d97593a68f7d97b2)