don't claim support for resolv.h APIs that aren't supported

the value 19991006 for __RES implies availability of res_ninit and
related functions that take a resolver state argument; these are not
supported since our resolver is stateless. instead claim support for
just the older API by defining __RES to 19960801.

based on patch by Dmitrij D. Czarkoff.

fix res_mkquery and res_send prototypes in resolv.h

historically these functions appeared in BSD 4.3 without prototypes,
then in the bind project prototypes were added to resolv.h, but those
were incompatible with the definitions of the implementation.
the bind resolv.h became the defacto api most systems use now, but the
old internal definitions found their way into the linux manuals and thus
into musl.

Fix dn_comp prototype and add stub

This function is used by ping6 from iputils.

de-duplicate dn_expand, fix return value and signature, clean up

the duplicate code in dn_expand and its incorrect return values are
both results of the history of the code: the version in __dns.c was
originally written with no awareness of the legacy resolver API, and
was later copy-and-paste duplicated to provide the legacy API.

this commit is the first of a series that will restructure the
internal dns code to share as much code as possible with the legacy
resolver API functions.

I have also removed the loop detection logic, since the output buffer
length limit naturally prevents loops. in order to avoid long runtime
when encountering a loop if the caller provided a ridiculously long
buffer, the caller-provided length is clamped at the maximum dns name
length.

add prototype for dn_skipname

header file fixes: multiple include guard consistency and correctness

one file was reusing another file's macro name, and many had
inconsistent underscores and application of SYS prefix, etc.

patch by Szabolcs Nagy (nsz)

remove useless "extern" keywords in headers

initial check-in, version 0.5.0