Closes https://github.com/MusicPlayerDaemon/MPD/issues/841

Mounting one storage URI twice on different mount points can lead to
conflicts with the database cache file, and it doesn't make a lot of
sense.

But most importantly, our udisks storage plugin will unmount the disk
from the kernel VFS, and if two exist, they will compete with each
others.  We could (and should) fix this in the udisks storage plugin,
but for now, this workaround is good enough (and useful).

When mounting something over a directory that is already a mount
point, CompositeStorage::Mount() silently overwrites the previously
mounted storage, disposing it.  After that, SimpleDatabase::Mount()
will fail and handle_mount() will roll back the
CompositeStorage::Mount() command, effectively unmounting what was
there before (and also leaking memory).

Closes https://github.com/MusicPlayerDaemon/MPD/issues/918

The ones in std have overloads for const char/char.
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>

The former has been deprecated by C++14. They are also the same.

From the standard:

The contents and meaning of the header<cinttypes>are the same as the C
standard library header<inttypes.h>, with the following changes:

-The header<cinttypes>includes the header<cstdint>instead of<stdint.h>,and
—if and only if the typeintmax_tdesignates an extended integer type
 (6.7.1), the following functionsignatures are added:intmax_t
abs(intmax_t);imaxdiv_t div(intmax_t, intmax_t);which shall have the same
semantics as the function signaturesintmax_t imaxabs(intmax_t)andimaxdiv_t
imaxdiv(intmax_t, intmax_t), respectively.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Only _WIN32 is defined by the compiler, and WIN32 is not standardized
and may be missing.

Closes #169

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

These events are relevant for all partitions.

Eliminate global variables.

Prepare for moving idle events to class Partition.  Right now, it's
just a wrapper for idle_add().