RFR: 8252324: Signal related code should be shared among POSIX platforms [v2]

[Gerard Ziemski]

> This is a fresh start for https://github.com/openjdk/jdk/pull/157
> 
> Please review this change that refactors common POSIX code into a separate
> file.
> 
> Currently there appears to be quite a bit of duplicated code among POSIX
> platforms, which makes it difficult to apply single fix to the signal code.
> With this fix, we will only need to touch single file for common POSIX
> code fixes from now on.
> 
> ---------
> ### Progress
> - [x] Change must not contain extraneous whitespace
> - [x] Commit message must refer to an issue
> - [ ] Change must be properly reviewed
> 
> 
> 
> ### Download
> `$ git fetch https://git.openjdk.java.net/jdk pull/497/head:pull/497`
> `$ git checkout pull/497`

Gerard Ziemski has updated the pull request incrementally with one additional commit since the last revision:

  Remove old comment about 4528190, remove empty lines in g_signal_info struct

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/497/files
  - new: https://git.openjdk.java.net/jdk/pull/497/files/06b104d0..e6c781e7

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=497&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=497&range=00-01

  Stats: 7 lines in 1 file changed: 0 ins; 6 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/497.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/497/head:pull/497

PR: https://git.openjdk.java.net/jdk/pull/497