Call posix_spawn instead of fork in ssh_execute_command

Xiang Xiaorequested to merge
xiaoxiang7812161/libssh-mirror:spawn into master

and call ssh_execute_command in ssh_exec_shell to avoid the code duplication

the change avoid to use fork which is hard to implement in MMUless system

Checklist

  • Commits have Signed-off-by: with name/author being identical to the commit author
  • Code modified for feature
  • Test suite updated with functionality tests
  • Test suite updated with negative tests
  • Documentation updated

Reviewer's checklist:

  • Any issues marked for closing are addressed
  • There is a test suite reasonably covering new functionality or modifications
  • Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTING.md
  • This feature/change has adequate documentation added
  • No obvious mistakes in the code

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