Major changes:
- Introduce SIGHUP reloading.
- The symlink device can be enabled by '--enable-symlink-device' configure
option.
- Fix the unknown error under some mutex condition.
- Fix the wrong reservation key handling.
- Use Clang as first choice compiler.
- Use _GNU_SOURCE on linux platform.
Current Limitations of SIGHUP support:
- The parameters other than PG, IG, and LU are not reloaded by SIGHUP.
- PG and IG mapped to LU can't be deleted by SIGHUP.
- If you delete an active LU, all connections of the LU are closed by
SIGHUP.
- Can't modify LU by SIGHUP.
- Updating IG is not affected until the next login.
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