Error out early if posix_spawnp(3) is called with an empty file name

Do as the manual says, and use _PATH_DEFPATH if PATH is not present in
the environment rather than simply turning into posix_spawn() in that case.

Also, we cannot use strtok() to parse PATH, the sem

Do as the manual says, and use _PATH_DEFPATH if PATH is not present in
the environment rather than simply turning into posix_spawn() in that case.

Also, we cannot use strtok() to parse PATH, the semantics don't fit the API.
Borrow the guts of execvp for the PATH search.

We still simply check for a file with 'x' permission, and assume that one
will do, whatever it is, which isn't really correct, but ...

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Add bunch of missing includes of namespace.h in libc

The NetBSD Standard C Library uses internally some of its functions with
a mangled symbol name, usually "_symbol". The internal functions shall n

Add bunch of missing includes of namespace.h in libc

The NetBSD Standard C Library uses internally some of its functions with
a mangled symbol name, usually "_symbol". The internal functions shall not
use the global (public) symbols.

This change eliminates usage of the global changes of the following symbols:
- strlcat -> _strlcat
- sysconf -> __sysconf
- closedir -> _closedir
- fparseln -> _fparseln
- kill -> _kill
- mkstemp -> _mkstemp
- reallocarr -> _reallocarr
- strcasecmp -> _strcasecmp
- strncasecmp -> _strncasecmp
- strptime -> _strptime
- strtok_r -> _strtok_r
- sysctl -> _sysctl
- dlopen -> __dlopen
- dlclose -> __dlclose
- dlsym -> __dlsym

Sponsored by <The NetBSD Foundation>

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Use C++ compatible declaration for posix_spawn (instead of the C99 specific
posix one).

Add userland part of posix_spawn. Libc functions imported from FreeBSD.
Based on Charles Zhang's summer of code project.