shared: split out crypt() specific helpers into its own .c/.h in src/shared/

This way we can use libxcrypt specific functionality such as
crypt_gensalt() and thus take benefit of the newer algorithms libxcrypt
implements. (Also adds support for a new env var $SYSTEMD_CRYPT_PREFIX
which may be used to select the hash algorithm to use for libxcrypt.)

Also, let's move the weird crypt.h inclusion into libcrypt.h so that
there's a single place for it.

src/basic/user-util.c

@@ -945,40 +945,3 @@ int fgetsgent_sane(FILE *stream, struct sgrp **sg) {
         return !!s;
 }
 #endif
-
-int make_salt(char **ret) {
-        static const char table[] =
-                "abcdefghijklmnopqrstuvwxyz"
-                "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
-                "0123456789"
-                "./";
-
-        uint8_t raw[16];
-        char *salt, *j;
-        size_t i;
-        int r;
-
-        /* This is a bit like crypt_gensalt_ra(), but doesn't require libcrypt, and doesn't do anything but
-         * SHA512, i.e. is legacy-free and minimizes our deps. */
-
-        assert_cc(sizeof(table) == 64U + 1U);
-
-        /* Insist on the best randomness by setting RANDOM_BLOCK, this is about keeping passwords secret after all. */
-        r = genuine_random_bytes(raw, sizeof(raw), RANDOM_BLOCK);
-        if (r < 0)
-                return r;
-
-        salt = new(char, 3+sizeof(raw)+1+1);
-        if (!salt)
-                return -ENOMEM;
-
-        /* We only bother with SHA512 hashed passwords, the rest is legacy, and we don't do legacy. */
-        j = stpcpy(salt, "$6$");
-        for (i = 0; i < sizeof(raw); i++)
-                j[i] = table[raw[i] & 63];
-        j[i++] = '$';
-        j[i] = 0;
-
-        *ret = salt;
-        return 0;
-}

src/basic/user-util.h

@@ -137,6 +137,4 @@ int fgetsgent_sane(FILE *stream, struct sgrp **sg);
 int putsgent_sane(const struct sgrp *sg, FILE *stream);
 #endif
 
-int make_salt(char **ret);
-
 bool is_nologin_shell(const char *shell);