bpo-20104 exposed os.posix_spawn(), but not os.posix_spawnp().

os.posix_spawnp() would be useful to support executable with no directory. See bpo-35537 "use os.posix_spawn in subprocess".

I'm not sure what is the best API:

* Add os.posix_spawnp()? duplicate the documentation and some parts of the C code (but share most of the C code)
* Add a new optional parameter to os.posix_spawn()? Ideas of names: 'use_path' or 'search_executable'. Internally, the glibc uses SPAWN_XFLAGS_USE_PATH flag to distinguish posix_spawn() and posix_spawnp().

execvp() uses the PATH environment variable, or use confstr(_CS_PATH) if PATH is not set. I guess that posix_spawnp() also uses confstr(_CS_PATH) if PATH is not set.

Currently, my favorite option is to add a new optional 'use_path' parameter to the existing os.posix_spawn() function.