sockaddr_un.sun_path should not use UNIX_PATH_MAX

(from bug 3156 comment 18) devchk complaint on the native value of
UNIX_PATH_MAX being undefined.

POSIX now says this:

      DESCRIPTION
            The <sys/un.h> header shall define the sockaddr_un structure, which
shall include at least the following members:

            sa_family_t sun_family Address family.
            char sun_path[] Socket pathname.

            The sockaddr_un structure is used to store addresses for UNIX
domain sockets. Values of this type shall be cast by applications
to struct sockaddr for use with socket functions.
            The <sys/un.h> header shall define the sa_family_t type as
described in <sys/socket.h>.

      APPLICATION USAGE
            The size of sun_path has intentionally been left undefined. This is
because different implementations use different sizes. For example, 4.3 BSD
uses a size of 108, and 4.4 BSD uses a size of 104. Since most implementations
originate from BSD versions, the size is typically in the range 92 to 108.

            Applications should not assume a particular length for sun_path or
assume that it can hold {_POSIX_PATH_MAX} bytes (256).

While our partly problematic definition is:

    struct sockaddr_un {
        sa_family_t sun_family; /* AF_UNIX */
        char sun_path[UNIX_PATH_MAX];
    };

seems like we have no particular need to define UNIX_PATH_MAX, we could just
match the generic glibc definition, which is:

    char sun_path[108]; /* Path name. */
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