stat: print "Block size" in file system mode

[Socialpranker]

stat -f builds its second line out of two independent word translations,
stat-word-block (Block) and stat-word-size (Size), so the label comes
out capitalized on both words:

$ stat -f /etc/passwd            # this build
  File: "/etc/passwd"
    ID: 1f5177f6d898479e Namelen: 255     Type: overlayfs
Block Size: 4096       Fundamental block size: 4096
Blocks: Total: 118523922  Free: 116434161  Available: 110395109
Inodes: Total: 30179328   Free: 29976433

GNU prints Block size: there — lowercase s, one label rather than two
words. Everything else on the line, including the %-10s padding and the
Fundamental block size: label that follows, already matches.

This PR replaces the Block + Size pair with a single localizable key,
stat-word-block-size-capitalized. Composing a label out of separate words is
also a translation hazard: the French locale currently renders Bloc Taille:,
and with the label as one string it becomes Taille bloc:. The existing
stat-word-block and stat-word-size keys stay — they are still used for
IO Block: and Size: in the non--f format.

Testing: new test_fs_default_format_block_size_label in
tests/by-util/test_stat.rs asserts the label; it fails on current main and
passes with this change (verified by reverting stat.rs and the locales alone
and re-running). The full stat suite is green (33 passed),
cargo fmt --all --check and cargo clippy -p uu_stat --all-targets are
clean. A release build of this branch was also compared against the system
stat inside debian:stable-slim for /, /etc, /etc/passwd, /tmp,
/proc and /dev — 6 of 6 byte-identical, where before all six differed on
this line.

Behaviour was established by observing GNU stat's output on a Debian system —
no GNU source was consulted.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/inotify-dir-recreate (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/printf/printf-surprise is now being skipped but was previously passing.
Congrats! The gnu test tests/tail/tail-n0f is now passing!