release v0.21.21

Found with modernize-use-nodiscard
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Signed-off-by: Rosen Penev <rosenp@gmail.com>

Found with modernize-use-default-member-init
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Found with readability-redundant-access-specifiers
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Documentation says the limit is 5, but it was really 10 (at least
since 2004).  But since MPD wants to promote using many small clients
idling around, and these clients consume only very few resources, it
seems reasonable to raise this limit's default value.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

Signed-off-by: Rosen Penev <rosenp@gmail.com>

Signed-off-by: Rosen Penev <rosenp@gmail.com>

Signed-off-by: Rosen Penev <rosenp@gmail.com>

Found with -Wdouble-promotion
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Merge remote-tracking branches 'neheb/fwhfggwe', 'neheb/nvm2', 'neheb/nvm22', 'neheb/bvm' and 'neheb/cl2'

Found with -Wunreachable-code-break
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Found with -Wextra-semi
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Signed-off-by: Rosen Penev <rosenp@gmail.com>

Found with clang's -Winconsistent-missing-destructor-override
Signed-off-by: Rosen Penev <rosenp@gmail.com>

[[maybe_unused]] (introduced in C++17) is standard C++.

https://clang.llvm.org/docs/AttributeReference.html#maybe-unused-unused
says that this is equivalent to the GNU unused attribute.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

[[nodiscard] (introduced in C++17) can be used instead.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

[[deprecated]] (introduced in C++14) may be used instead.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Signed-off-by: Rosen Penev <rosenp@gmail.com>

Found with bugprone-incorrect-roundings
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Boost does not seem to offer an overload for lrint.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

exp10 is a GNU function, is not part of C++, and is not available
everywhere.

pow(10,x) is an alternative that works just as well. It is used in musl as
the implementation of exp10.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

This is the case with uClibc-ng currently.
Signed-off-by: Rosen Penev <rosenp@gmail.com>