ui: Remove/adapt some old code for a big UI refactor

- Remove abstract_ui global, now it is always active
- Remove some terminal handling code
- Remove unused functions
- Remove HAVE_TGETENT/TERMINFO/TERMIOS/IOCTL #ifdefs
- Remove tgetent/terminfo from version.c
- Remove curses/terminfo dependencies
- Only start/stop termcap when starting/exiting the program
- msg_use_printf will return true if there are no attached UIs(
  messages will be written to stdout)
- Remove `ex_winpos`(implement `:winpos` with `ex_ni`)

src/nvim/sha256.c

@@ -351,40 +351,3 @@ bool sha256_self_test(void)
   }
   return failures == false;
 }
-
-/// Fill "header[header_len]" with random_data.
-/// Also "salt[salt_len]" when "salt" is not NULL.
-///
-/// @param header
-/// @param header_len
-/// @param salt
-/// @param salt_len
-void sha2_seed(char_u *restrict header, size_t header_len,
-               char_u *restrict salt,   size_t salt_len)
-{
-  static char_u random_data[1000];
-  char_u sha256sum[SHA256_SUM_SIZE];
-  context_sha256_T ctx;
-
-  unsigned int seed = (unsigned int) os_hrtime();
-
-  size_t i;
-  for (i = 0; i < sizeof(random_data) - 1; i++) {
-    random_data[i] = (char_u) ((os_hrtime() ^ (uint64_t)rand_r(&seed)) & 0xff);
-  }
-  sha256_start(&ctx);
-  sha256_update(&ctx, random_data, sizeof(random_data));
-  sha256_finish(&ctx, sha256sum);
-
-  // put first block into header.
-  for (i = 0; i < header_len; i++) {
-    header[i] = sha256sum[i % sizeof(sha256sum)];
-  }
-
-  // put remaining block into salt.
-  if (salt != NULL) {
-    for (i = 0; i < salt_len; i++) {
-      salt[i] = sha256sum[(i + header_len) % sizeof(sha256sum)];
-    }
-  }
-}