Move sha1.[ch] to crypt.[ch] and make crypto API independent of sha1.

This patch moves all public functions related to sha1 to crypt.c. This
allows to remove sha1.c, sha1.h and hash.h. It also removes the
HASH_TYPE define and avoids the word "sha1" in the public API and
its callers. The former sha1_hash() is now called hash_function().