MFC r276564, r276654:

Cast *path to silence clang -Wpointer-sign warning.

Instead of hiding the kern___getcwd() bug by bogus cast
in r276564, change path type to char * (pathnames are always char *).
And remove bogus casts of malloc().
kern___getcwd() internally doesn't actually use or support u_char *
paths, except to copy them to a normal char * path.

These changes are not visible to libc as libc/gen/getcwd.c misdeclares
__getcwd() as taking a plain char * path.

While here remove _SYS_SYSPROTO_H_ for __getcwd() syscall as
we always have sysproto.h.