Handle ENOSYS subprocess errors

[sheredom]

Summary

• add subprocess_error_not_supported as a portable create error
• map POSIX ENOSYS to the new error
• return the new error when custom working directories are unavailable while preserving errno == ENOSYS
• document and test the behavior

Testing

• standard test suite
• AddressSanitizer test suite
• UndefinedBehaviorSanitizer test suite
• targeted unsupported-CWD tests with SUBPROCESS_HAVE_CWD=0 (all supported C/C++ language modes)

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