Full posix_spawn support

[bjorn3]

The standard library uses posix_spawn where possible, so this is necessary for spawning processes from rust nowadays. Some of these functions already have a stub implementation, but many are missing and those that have a stub implementation will need to actually be implemented.

• posix_spawn
• posix_spawnattr_destroy
• posix_spawnattr_getflags
• posix_spawnattr_getpgroup
• posix_spawnattr_getsigdefault
• posix_spawnattr_getsigmask
• posix_spawnattr_init
• posix_spawnattr_setflags
• posix_spawnattr_setpgroup
• posix_spawnattr_setsigdefault
• posix_spawnattr_setsigmask
• posix_spawn_file_actions_addchdir
• posix_spawn_file_actions_addchdir_np
• posix_spawn_file_actions_addclose
• posix_spawn_file_actions_adddup2
• posix_spawn_file_actions_addopen
• posix_spawn_file_actions_destroy
• posix_spawn_file_actions_init
• posix_spawnp