libcasper: Fix inconsistent error codes of cap_get{addr,name}info()

[hrs]

Summary

The get{addr,name}info(3) API is designed to return an API-specific error
code that is independent of errno. The cap_get{addr,name}info() functions
returned either an errno or API-specific error code inconsistently.
This change fixes this mismatch.

When the API returns an errno, the return value itself is set to
EAI_SYSTEM and errno is set depending on the actual error. So, usually
this API is called in the following form:

    error = getnameinfo(...);
    if (error == EAI_SYSTEM)
	perror("getnameinfo");
    else if (error)
        errx(1, "getnameinfo: %s", gai_strerror(error);

If the above getnameinfo() call is replaced with cap_getnameinfo(),
it breaks the error handling. For example, the cap_get{addr,name}info()
functions can return ENOTCAPABLE.

This change simply adds "errno", in addition to "error", to the nvlout and
cap_get{addr,name}info() restores the errno if the error code is EAI_SYSTEM.

[oshogbo]

lib/libcasper/services/cap_net/cap_net.c

369

Using dnvlist should be a little bit faster, because we don't have to travel nvlist twice (for exists and get).

serrno = dnvlist_get_number(nvl, "errno", 0);

[hrs]

Update the diff to use dnvlist_get_number().

[hrs]

lib/libcasper/services/cap_net/cap_net.c

369

Thanks! Updated the diff.