This commit adds support in if_indextoname() to return ENXIO when
no interface is found. This conforms to the Open Group/RFC 3493
specification
In order to leverage errno set support, the set_errno macro from
sockets.c was moved to sockets_priv.h
Couple of more cleanups for task #14314 involving includes:
1) if.h name should match if_api.c due to LwIP convention and history.
Standard if.h include can be used with compatibility header in
posix/net/if.h
2) API header (if.h) should not be included in core code. This include
has been eliminated by moving the definition of IF_NAMESIZE to
netif.h as NETIF_NAMESIZE. This is now the canonical definition
and IF_NAMESIZE just maps to it to provide the standard type
This renames if.c to if_api.c in order to avoid using a common source
file name which may collide with other source files from non-LwIP
projects. See discussion in task #14314
